US Vice-President Joe Biden has said Russia must "stop talking and start acting" to defuse the Ukraine crisis.
He was speaking after meeting interim PM Arseniy Yatsenyuk in Kiev.
Mr Biden warned Russia that further "provocative behaviour" would lead to "greater isolation" and urged Moscow to end its alleged support for pro-Russian militants in eastern Ukraine.
Separately, a Ukrainian military plane was hit by small arms fire over eastern Ukraine, the defence ministry says.
The aircraft suffered minor damage over Sloviansk, which is held by pro-Russian militants, when it was targeted by automatic gunfire, according to the ministry. No-one was hurt and the plane returned safely to Kiev.
"Thanks to professional actions, military pilots managed to land the plane at the airfield without consequences," the ministry's statement said.
The Antonov An-30 aerial survey plane was carrying out surveillance at the time.
The funerals have meanwhile taken place of three men shot on Sunday during a raid on a checkpoint manned by pro-Russian separatists near Sloviansk.
The local separatists said the attack was carried out by ultra-nationalist Right Sector militants but Kiev called it a "provocation" staged by Russian special forces.
The bodies of those killed lay in open coffins at the funeral ceremony at the Church of the Holy Spirit in the centre of Sloviansk.
Earlier in remarks to Ukrainian MPs, Mr Biden said the US stood with Ukraine's new leaders against "humiliating threats" - an apparent reference to Russia.
The vice-president called on Moscow to urge the pro-Russian separatists to leave the buildings they are occupying in eastern Ukraine, and to abandon checkpoints.
Mr Biden also stressed the need for the new authorities to tackle corruption, adding: "The opportunity to generate a united Ukraine, getting it right is within your grasp."
The US is to provide an additional $50m for political and economic reforms in Ukraine, including $11m to help run the presidential election due on 25 May.
An further $8m is being provided for non-lethal military assistance, such as bomb disposal equipment and radios.
After meeting Mr Biden, interim PM Yatsenyuk accused Russia of behaving "as gangsters in the modern century".
Russian President Vladimir Putin on Monday signed a decree to rehabilitate Crimea's Muslim Tatars and other ethnic minorities who suffered during the rule of Soviet dictator Josef Stalin.
After a referendum in Crimea last month, the territory was incorporated into Russia, something Mr Biden said the US would never recognise.
Crimea's Tatar community opposed the peninsula's takeover by Russia. On Tuesday, the Tatar assembly said the leader of the community, Mustafa Dzhemilev, had been banned from returning to Crimea for five years, along with his deputy.
'Men in masks'
Moscow and Washington are accusing each other of breaking last week's Geneva accord on resolving the Ukraine crisis, and the US is planning further sanctions should Russia fail to fulfil its Geneva commitments.
But Russian Prime Minister Dmitry Medvedev told the Russian parliament on Tuesday that Russia would be able to "minimise the consequences" of any further sanctions.
The 17 April Geneva accord stipulated an immediate end to violence in eastern Ukraine and called on illegal armed groups to surrender their weapons and leave official buildings.
Pro-Russian militants are still holding official buildings in at least nine towns and cities in the Donetsk region.
In Kramatorsk, another building - a police station - was seized on Tuesday.
Mr Biden again accused Russia of supporting "men in masks in unmarked uniforms" who the US says are directing pro-Russian activity in the East.
Moscow denies being behind the protests and seizures of buildings.
However, Ukraine says photos released by the Ukrainian government and distributed by the US State Department show Russian soldiers among militants holding official buildings in eastern Ukraine.
There was no immediate response to the pictures from the Russian government.
Ukraine has been in turmoil since last November, when Kiev was gripped by protests over whether the country should lean more towards Russia or Europe.