US Secretary of State John Kerry has accused pro-Russian separatists in east Ukraine of a "blatant land grab".
He was speaking after reports that the rebels had extended the area they control, violating a ceasefire plan.
Ukraine says Russia has more than 9,000 soldiers fighting alongside the rebels, a claim it denies.
Meanwhile, foreign ministers from Ukraine, Russia, France and Germany issued a joint call to end the fighting, following talks in Berlin.
Speaking after the meeting, German Foreign Minister Frank-Walter Steinmeier said it was not a breakthrough "but I think we saw tangible progress".
He also said they had agreed on a procedure for pulling back heavy weapons 15km (nine miles) from a demarcation line defined in last year's Minsk agreement.
His Russian counterpart Sergei Lavrov said, quoted by the Russian Interfax news agency, that if heavy weapons were withdrawn "we'll be able to speak of a substantial de-escalation of the current conflict".
'Occupation bid'
Mr Kerry said the recent upsurge in fighting was "an alarming situation" adding that the US was "particularly concerned" by rebel moves to "attempt to gain control of a very significant rail juncture" in eastern Ukraine.
He said there had been a large extension of the line of control that separates rebel-held territory from the rest of Ukraine.
"This is a blatant land grab and is in direct contravention to the Minsk [ceasefire] agreements which they signed up to," he added.
The US ambassador to the UN, Samantha Power, accused Russia of escalating the violence.
She said a new peace proposal by Russian President Vladimir Putin was little more than an attempt at military occupation.
"The plan would seek to legitimise territorial gains made by separatists in September as well as Russian personnel and equipment on the territory of Ukraine," she told a special meeting of the UN Security Council.
"Let us pull the veil away from Putin's peace plan and call it for what it is - a Russian occupation plan."
Mr Putin put forward the proposal last week but the Kremlin said Ukrainian President Petro Poroshenko had rejected it.
A spokesman for Mr Putin said the proposal called for a ceasefire and the withdrawal of heavy artillery by both sides.
But Ms Power said the plan would free Russia from a commitment it made in Minsk, Belarus, last September to withdraw its fighters and return the control of the international border to Ukraine.
Airport 'abandoned'
More than 4,800 people have been killed and some 1.2 million displaced since pro-Russian rebels seized parts of Luhansk and Donetsk regions in April.
The move followed Russia's annexation of Ukraine's Crimea peninsula in March.
Some of the fiercest clashes in recent days have centred on the airport in Donetsk city, which government troops and rebels have been fighting over for months.
Unconfirmed reports late on Wednesday said defending Ukrainian forces had abandoned the ruined complex.
There are also reports of a fresh separatist advance in an area north-west of the rebel-held city of Luhansk. Fighting is said to be centred on two checkpoints along a main road.
On Wednesday, President Poroshenko said Russia had more than 9,000 troops and 500 tanks, heavy artillery and armoured personnel carriers in eastern Ukraine.
Speaking at the World Economic Forum in Davos, Switzerland, he said: "If this is not aggression, what is aggression?"
(BBC)
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