The United States and Russia clashed over the crisis in Ukraine as OSCE foreign ministers met in Vienna Thursday, casting doubt on efforts to negotiate terms for a U.N. peacekeeping force.

The regional body has 57 members, but all eyes were on U.S. Secretary of State Rex Tillerson and Russia's Foreign Minister Sergei Lavrov, who were due to meet one-on-one later in the day.

At stake are efforts to end the brutal war between Kiev government forces and Russian-backed separatists in eastern Ukraine by deploying a United Nations force to protect the OSCE's unarmed monitoring mission.

Moscow and Washington both back such a mission in principle but disagree over its mandate, and as ministers sat down together in Vienna's magnificent Hofburg Palace there was little sign of a breakthrough.

"We've reached an absolute low point regarding confidence between the main players," Thomas Greminger, the Organization for Security and Co-operation in Europe's secretary general, admitted as talks began.

After long resisting the idea, Russia now wants a U.N. peacekeeping force to help end the war between Kiev and Russian-backed separatists in the east of the country that has killed more than 10,000 people since 2014. But under Russia's vision, the force will have a limited mandate to protect the OSCE's ceasefire monitors.

Western powers, led by the United States, want a force with a robust mandate that would allow it to protect the 600 OSCE monitors there, police ceasefire lines and investigate ceasefire breaches across eastern Ukraine. They fear that a United Nations mission that only polices the front line would serve to create a frozen conflict that would de facto lock in Russian gains from its intervention in Ukraine.

Addressing the opening OSCE session, Lavrov accused Western powers of seeking to "disrupt specific consideration" of a draft U.N. Security Council resolution to set up a U.N. force to escort the OSCE under its current mandate.

The U.S. idea of a robust force, he argued, would amount to "an occupational administration ... in order to bury a package of measures unanimously approved by the UN Security Council and to solve this problem by force."

Tillerson did not directly address the peacekeeping issue -- which is not a matter for the OSCE to decide -- but was strident in his condemnation of the threat he said Russia poses to the existing unarmed mission.

"Of all the challenges confronting the OSCE today, none is more challenging nor vexing than the situation in Ukraine," he warned, stressing the US will never lift sanctions until Russia returns control of Crimea and the disputed Donbass region back to Ukraine.

Tillerson noted that more civilians were killed this year in eastern Ukraine than in 2016, and that ceasefire violations are up 60 percent. "We should be clear about the source of this violence. Russia is arming, leading, training and fighting alongside anti-government forces," he said. "We call on Russia and its proxies to end the harassment, intimidation and its attacks on the OSCE special monitoring mission," he demanded. He paid tribute to an American paramedic who was killed in April when his OSCE patrol hit a landmine in a separatist-occupied area.