US Secretary of State John Kerry and Russian Foreign Minister Sergei Lavrov have arrived in Paris for crisis talks on Ukraine.
The meeting was hastily arranged after President Vladimir Putin phoned President Barack Obama on Friday.
Russia has annexed Crimea and there are reports of thousands of Russian troops massed close to Ukraine’s borders.
Earlier Sergei Lavrov set out demands for a neutral and federal Ukraine, an idea Kiev called “full capitulation”.
However, Sergei Lavrov has categorically denied any plans for an invasion.
However, the Russian foreign minister has stressed Moscow will protect the rights of ethnic Russians and Russian speakers, after pro-EU protests in Kiev led to the ousting of Ukraine’s President Viktor Yanukovych.
On Sunday, the US ordered its top general in Europe to return early from a trip to Washington.
NATO’s supreme allied commander Europe, General Philip Breedlove, had been due to testify to Congress, but a Pentagon spokesman told Reuters his return was prudent “given the lack of transparency and intent from Russian leadership about their military movements across the border”.
Hours before the Paris talks were due to take place at the Russian ambassador’s residence, Sergei Lavrov told Russian state TV that Ukraine should come up with a new constitution “providing for a federal structure” and neutrality.
The Russian foreign minister said Moscow, the US and EU should act as a support group for Kiev to begin a nationwide dialogue that did not involve the “armed radicals”. Moscow claims that fascists have taken power in Ukraine, jeopardizing the safety of Russian speakers.
In an interview on Saturday, Sergei Lavrov said Russia had been deceived after being promised “there would be no movement of NATO military infrastructure closer to our borders”.
The Ukrainian foreign ministry said it deeply regretted Sergei Lavrov’s “patronizing” remarks.
“At the point of its automatic rifles, this aggressor demands only one thing – Ukraine’s full capitulation, its split and the destruction of Ukrainian statehood,” said a statement carried by Interfax-Ukraine news agency.
NATO’s outgoing Secretary-General Anders Fogh Rasmussen warned on Sunday that Russia’s government was “[flouting] the principle that every state is sovereign and free to choose its own fate”.
Vladimir Putin is also thought to be demanding that Washington accepts Crimea’s independence from Ukraine.
Separately, Moscow is keen to tackle the issue of Trans-Dniester, a pro-Russian separatist region of Moldova on the south-western border of Ukraine. It accuses Ukraine and Moldova of “blockading” the area while the EU and the US stay silent.
US officials are divided over whether Vladimir Putin is seeking to ease tensions or is still planning further military action.
The Pentagon believes Moscow has massed tens of thousands of troops close to Ukraine’s eastern border.
Food, medicines and a field hospital are said to be among the supplies moved into position, officials say, which would not be necessary for any spring military exercise.
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