Diplomatic efforts to defuse tensions over Ukraine in the German capital of Berlin yielded no concrete results, Ukrainian and Russian representatives said separately on Friday, as top U.S. officials resumed their warnings that the Kremlin may give the order for invasion any time now.

U.S. Secretary of State Antony Blinken on Friday said Russia is amassing yet more troops on Ukraine's border, and warned an invasion during the ongoing Winter Olympics was possible.

"We're in a window when an invasion could begin at any time, and to be clear that includes during the Olympics," he said, brushing aside suggestions that Moscow would wait until the Beijing Games have concluded to avoid upstaging its ally China.

"Simply put, we continue to see very troubling signs of Russian escalation," Blinken said after a meeting of counterparts from the so-called Quad countries – Australia, India, Japan and the United States – in Melbourne.

Earlier on Thursday, U.S. President Joe Biden urged Americans to immediately leave Ukraine, as Russia's live-fire drills and buildup of troops around the ex-Soviet state deepened fears of an invasion. Some U.S. estimates say some 130,000 Russian soldiers are grouped in dozens of combat brigades near the border with Ukraine.

"American citizens should leave now," Biden said in a pretaped interview with NBC News.

"We're dealing with one of the largest armies in the world. It's a very different situation and things could go crazy quickly."

Biden reiterated that under no circumstances would he send U.S. troops to Ukraine, even to rescue Americans in case of a Russian invasion.

"That's a world war. When Americans and Russians start shooting one another, we're in a very different world," he said.

Ukraine has been plagued by conflict in its eastern regions since March 2014 following Russia’s invasion and annexation of Crimea. Parts of Ukraine's Luhansk and Donetsk regions along the Russian border have been controlled by pro-Russian separatists for nearly eight years since the conflict started in 2014.

Normandy format talks resume without way out

Negotiations between political advisers from Ukraine, Russia, France and Germany under the Normandy format ended without yielding any substantial results, Ukrainian presidential sources said early Friday.

They have agreed to meet again in March after "difficult talks" in Berlin, sources close to French and German negotiators told (Agence France-Presse) AFP on Friday.

The second meeting of 2022, in which Ukraine was represented by Andriy Yermak, the head of the Office of the President, lasted over nine hours, said the Ukrainian presidency in a statement.

Reiterating Ukraine’s commitment to a political and diplomatic settlement of the ongoing tensions, Yermak said the country would continue to take measures to intensify the work of all existing negotiation formats in order to facilitate the peace process. Yermak said there had been a complete cease-fire for several days last week, which he called a "very strong result."

The mediators support the peace plan but "to our regret" are not pressing the Ukrainian government to fulfill its part under the Minsk agreement, The Kremlin’s Deputy Chief of Staff Dmitry Kozak said after nine hours of talks.

"It has not been possible to overcome the differences of opinion," said Kozak. He said the Ukrainian side interpreted the peace plan differently to the Russian side.

Kozak again criticized Ukraine's government for rejecting dialogue with separatist leaders in the eastern Ukrainian Luhansk and Donetsk regions and said the implementation of the Minsk agreement is a prerequisite for solving the conflict without violence.

All parties reaffirmed their willingness to continue negotiations in the Normandy format, the statement noted, adding the parties also agreed on the necessity to unblock the work of the Trilateral Contact Group.

Jens Ploetner and French President Emmanuel Macron’s diplomatic adviser Emmanuel Bonne were the other participants of the meeting who represented their countries.

A previous meeting of the Normandy format group was held two weeks ago in Paris, the first such meeting of the political advisers after more than a year’s break, amid growing tensions in the region. Russia recently amassed over 100,000 troops on its western border with Ukraine, stoking fears in the West of an invasion.

Moscow has denied that it is preparing to invade and said its troops are there for exercises. The Kremlin also issued a list of security demands from the West, including a rolling back of troop deployments to some ex-Soviet states and guarantees that Ukraine and Georgia will not join NATO.

Western leaders have been shuttling to Moscow in an effort to keep the lines of communication open, giving Russia a chance to air its grievances about NATO's expansion into eastern Europe and ex-Soviet states. But they have also sought to project their resolve in the face of what they say is a Russian escalation of an already-tense situation.

"Russia should not underestimate our unity and determination as a partner in the EU and as an ally in NATO," German Chancellor Olaf Scholz warned.

In a bid to "reduce chances of miscalculation" during the drills, U.S. and Belarusian defense chiefs held rare telephone talks, the Pentagon said Thursday.

British Foreign Secretary Liz Truss was the latest Western diplomat to travel to Moscow on Thursday, where she reported receiving promises from her Russian counterpart Sergei Lavrov that the Kremlin had no plans to invade Ukraine.

"We need to see those words followed up by actions," she told reporters after the talks.

But Lavrov said he was "disappointed" by the talks, saying the military drills and the movement of troops across Russia's own territory had spurred "incomprehensible alarm and quite strong emotions from our British counterparts and other Western representatives."

Truss' trip came just days after French President Emmanuel Macron conducted a round of shuttle diplomacy between Moscow and Kyiv, before briefing Scholtz about progress in Berlin.

The German chancellor will travel to Kyiv and Moscow next week for separate meetings with the Ukrainian and Russian leaders – including his first in-person meeting with Putin. His position on the new Nord Stream 2 natural gas pipeline from Russia to Germany will be under particularly close scrutiny.

In Washington this week, Scholz had been largely evasive about Biden's pledge to "bring an end" to the critical energy link should Russia invade Ukraine.

The chancellor later said it was a conscious decision "not to publish the entire catalog" of potential sanctions "because we can gain a little bit of power" by remaining vague.

The flurry of diplomatic activity included a meeting between British Prime Minister Boris Johnson and Stoltenberg.

"The number of Russian forces is going up. The warning time for a possible attack is going down," Stoltenberg said at a news conference with Johnson.

"Renewed Russian aggression will lead to more NATO presence, not less," he added.

But Johnson stressed after a meeting with Polish President Andrzej Duda – one of Ukraine's strongest allies in Europe – that Western states must "tirelessly pursue the path of diplomacy."

Troop buildup, drills resume

Russia has also sent six warships through the Bosporus for naval drills on the Black Sea and the neighboring Sea of Azov. Kyiv condemned their presence as an "unprecedented" attempt to cut off Ukraine from both seas.

Moscow and Minsk have not disclosed how many troops are participating, but the United States has said around 30,000 soldiers were being dispatched to Belarus from locations including Russia's Far East.

NATO Secretary-General Jens Stoltenberg said last week that Russia was expected to have 30,000 troops in Belarus as well SU-35 fighter jets, S-400 air defense systems and nuclear-capable Iskander missiles.

Russia's defense ministry insisted the exercises would center around "suppressing and repelling external aggression" and the Kremlin has promised the troops will go home after the exercises.

But Ukraine's President Volodymyr Zelenskyy said "the accumulation of forces at the border is psychological pressure from our neighbors."

Russia held a briefing for military attachés that lasted just eight minutes, and gave notice of an exercise that was already underway, a senior U.S. State Department official said.

"That's highly inconsistent with agreements for transparency for large military exercises in Europe. That's bad news," the official said.

Kyiv has launched its own military drills expected to mirror Russia's games on Thursday, but officials have said little about them out of apparent fear of escalating tensions. Like Moscow's joint drills with Minsk, Ukrainian drills will run until Feb. 20.

The Ukrainian forces, whose numbers have not been disclosed, are set to use Bayraktar drones and anti-tank Javelin and NLAW missiles provided by foreign partners. Kyiv was due to receive a further shipment of U.S. military aid later on Thursday.

Russia is seeking written guarantees that NATO will withdraw its presence from eastern Europe and never expand into Ukraine. The United States and NATO have officially rejected Russia's demands.

Washington has however floated the idea of the sides striking a new disarmament agreement for Europe – an offer viewed as dramatically insufficient by Moscow.