Vienna (AFP) -

The United States warned Russia on Thursday that its ties with the West can never recover until it pulls its forces out of Ukraine, as OSCE foreign ministers met in Vienna.

Diplomats had hoped that meetings on the sidelines of the 57-nation body would make progress on agreeing terms for a UN peacekeeping force for war-torn Ukraine.

But the major powers in the dispute are still far apart, and US Secretary of State Rex Tillerson went so far as to declare that the stand-off over Ukraine was the single most important obstacle to warmer US relations with Moscow.

Tillerson met Foreign Minister Sergei Lavrov behind closed doors, but when they had addressed the OSCE (Organization for Security and Co-operation in Europe) council earlier in the day, they traded allegations about interference in Ukraine and there appeared little room for compromise.

At a news conference with OSCE host Austria's Foreign Minister Sebastian Kurz, Tillerson recalled that US President Donald Trump had been clear during his campaign last year that he wanted better relations with Russia.

"But the issue that stands in the way is Ukraine," Tillerson stressed.  

"We can have differences in other arenas... but when one country invades another that is a difference that is hard to look past or to reconcile," he warned.

"And we've made this clear to Russia from the very beginning: That we must address Ukraine, it stands as the single most difficult obstacle to us renormalizing relations."

After the meeting later with Lavrov, Tillerson would not be drawn on what had been said but suggested that he was receiving unspecified commitments from the Russians.

"We are. I'm not going to tell you specifically what we get, but we get progress, that's what we get. We get dialogue, we get cooperation. We don't have it solved. You don't solve it in one meeting," Tillerson said.

- Blue helmets -

The conflict in eastern Ukraine between Kiev's government forces and Russian-backed separatists has already left 10,000 dead since 2014.

Neal Walker, UN humanitarian coordinator, said in Geneva on Thursday that the humanitarian situation was "worse than it has never been".

People "are now forced to make impossible choices between food, medicine, shelter, heating or their children's education. Millions of women, children and the elderly are daily at risk of shelling, intensive fighting and other hostilities," he said.

In principle, both Moscow and Washington have now expressed support for the idea of a UN peacekeeping mission.

But Russia envisions a limited force to simply protect the existing 600-strong group of unarmed OSCE observers, who are struggling to monitor a shaky ceasefire.

Washington, in contrast, has in mind a much more robust UN mission that would police ceasefire lines and investigate ceasefire breaches across eastern Ukraine, building the conditions for lasting peace. 

Western powers fear that a limited United Nations mission would serve to create a frozen conflict that would lock in Russian gains from its intervention in Ukraine.

But Lavrov, in his opening remarks to the OSCE ministerial council, argued that the bigger mission idea could become "an occupational administration" and be tantamount to attempting "to solve this problem by force".

- 'Vexing' situation -

Tillerson did not directly address the peacekeeping issue in his remarks, but was strident in his condemnation of the threat he said Russia poses to the existing unarmed OSCE 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 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.

Tillerson also paid tribute to an American paramedic killed in April when his OSCE patrol hit a landmine, and met a handful of other monitors, including one injured by a hand grenade, in Vienna.