Russian troops on Wednesday pressed their assault on a factory city in an attempt to seize parts of eastern Ukraine as U.S. President Joe Biden said Washington would supply more advanced rockets to Kyiv to help it force Moscow to negotiate an end to the war.

The battle for Severodonetsk has grown in intensity in recent days, with heavy casualties on both the Ukrainian and Russian sides. One of the industrial hubs on Russia's path to taking the eastern Luhansk region, Severodonetsk has become a target of massive Russian firepower since the failed attempt to capture Kyiv.

The Russians now control most of the destroyed city, according to regional authorities. Ukrainian forces are holding just 20% of the eastern city but there is still hope that they can prevent Russia taking full control, the head of the city administration told Reuters in a telephone interview on Wednesday.

Russian forces now control about 70% of Severodonetsk, said regional governor Serhiy Gaidai. He has previously said the city has been largely reduced to rubble and that Russian shelling has made it impossible to deliver aid or evacuate people.

"Some Ukrainian troops have retreated to more advantageous, pre-prepared positions," Gaidai said on the Telegram messaging app. He said Lysychansk was easier to defend because it is located on a hill but that Russian forces would target it with artillery and mortars once in full control of Severodonetsk.

The leader of the pro-Moscow Luhansk People's Republic, Leonid Pasechnik, told Tass news agency that Russian proxies had advanced slower than expected to safeguard city infrastructure and "exercise caution around its chemical factories."

Gaidai has urged Severodonetsk residents not to leave bomb shelters due to what he said was a Russian airstrike on a nitric acid tank. The Luhansk People's Republic's police force accused Ukraine's forces of damaging it.

Ukraine's General Staff said Russian forces, now 98 days into their invasion, were pounding infrastructure in eastern and southern regions including the symbolically important industrial city of Severodonetsk, the main focus of Moscow's offensive in recent days. It also said Russian forces continued to pound northern, southern and eastern districts of Severodonetsk, in Luhansk, one of two provinces in the eastern Donbass region that Moscow claims on behalf of separatists.

Meanwhile, Biden announced the supply of precision rocket systems and munitions that could strike at long-range Russian targets, part of a $700 million weapons package expected to be unveiled on Wednesday.

"We have moved quickly to send Ukraine a significant amount of weaponry and ammunition so it can fight on the battlefield and be in the strongest possible position at the negotiating table," Biden wrote in an opinion piece in the New York Times.

A senior Biden administration official said the new supplies – which come on top of billions of dollars worth of other equipment such as drones and anti-aircraft missiles – included the M142 High Mobility Artillery Rocket System (HIMARS), which Kyiv has said is "crucial" to counter Russian missile attacks.

Moscow assessed the new U.S. aid package "extremely negatively," Russia's Deputy Foreign Minister Sergei Ryabkov told state news agency RIA Novosti.

Addressing concerns that such weapons could draw the United States into direct conflict, senior administration officials said Ukraine gave assurances the missiles would not be used to strike inside Russia.

"These systems will be used by the Ukrainians to repel Russian advances on Ukrainian territory, but they will not be used on targets in Russian territory," one U.S. official said.

Russia on Wednesday sharply criticized the U.S. decision to supply advanced rocket systems and munitions to Ukraine, warning of an increased risk of direct confrontation with Washington.

Kremlin spokesperson Dmitry Peskov told reporters: "We believe that the United States is purposefully and diligently adding fuel to the fire."

When asked how Russia would respond if Ukraine used U.S.-supplied rockets to strike Russian territory, Peskov said: "Let's not talk about worst-case scenarios."

Nuclear forces

Shortly after the U.S. decision was announced, the Russian Defense Ministry said Russia's nuclear forces were holding drills in the Ivanovo province, northeast of Moscow, the Interfax news agency reported.

Some 1,000 troops were exercising in intense maneuvers using more than 100 vehicles including Yars intercontinental ballistic missile launchers, it cited the ministry as saying.

There was no mention of the U.S. decision to supply new weapons in the Interfax report.

Russia has also completed testing of its hypersonic Zircon cruise missile and will deploy it by the end of the year on a new frigate of its Northern Fleet, a senior military officer said on Wednesday.

If Russia captures Severodonetsk, and its smaller twin Lysychansk on the higher west bank of the Donets River, it will hold all of Luhansk, a key war aim of President Vladimir Putin's forces.

Weapons package

Ukraine says weapons sent by the United States and other countries since the start of the invasion have helped fend off Russian gains.

The new U.S. package includes ammunition, counterfire radars, a number of air surveillance radars, additional Javelin anti-tank missiles as well as anti-armor weapons, officials said.

German Chancellor Olaf Scholz, accused by critics of dragging his feet in helping Ukraine, said on Wednesday Berlin would supply Kyiv with its IRIS-T medium-range surface-to-air defense system.

Ukrainian President Volodymyr Zelenskyy has called for more weapons while lambasting the European Union, which agreed on Monday to cut imports of Russian oil, for not sanctioning energy from Russia sooner.

Pope Francis appealed on Wednesday for all blockades on wheat exports from Ukraine to be lifted, saying grain should not be used as a "weapon of war."

Putin launched his "special operation" on Feb. 24 to disarm and "denazify" Ukraine. Ukraine and its Western allies call this a baseless pretext for a war to seize territory.

Ukraine accuses Russia of war crimes on a huge scale, flattening cities and killing and raping civilians. Russia denies the accusations.