The US vice-president has said America will never recognise Russia's takeover of Crimea.
Speaking in Kiev, Joe Biden said nobody in the world would accept that Crimea now formed part of Russia's national territory.
He called on Russia to honour the terms of last week's agreement and encourage armed rebels to return control of State buildings in Ukraine.
And he says Russia must now act immediately, ""We've heard a lot from Russian officials in the past few days. But now it's time for Russia to stop talking and to start acting - act on the commitments that they made, to get pro-Russian separatists to vacate buildings and checkpoints, accept amnesty and address their grievances politically."
He addressed politicians at the Ukraine parliament in a show of support for the interim government and announced new technical support measures for the government on energy and economic reforms.
And he warned Russia against further provocative action after an agreement in Geneva last week.
He said: "There are some who are trying to pull Ukraine apart. Ukraine is in a struggle for its very future.
"Ukraine is an must remain one country - one united Ukraine.
"The US supports a strong united Ukraine with productive and peaceful relationships with both the east and the west, with both Russia and Europe.
"No nation has the right to simply grab land from another nation. We will never recognise Russia's illegal occupation of Crimea and neither will the world.
"More provocative behaviour by Russia will lead to more costs and greater isolation."
New supports
Among the new support measures was an additional $50m for political and economic reforms, and another $8m for non-lethal aid to Ukraine including vehicles and radios.
Mr Biden also called on Russia to withdraw troops from its borders with Ukraine and to "stop talking and start acting" on the surrender of militants in the east of the country.
His visit comes as pro-Russian separatists show no sign of surrendering government buildings seized in the predominantly Russian speaking east.
US and EU officials have said they will impose new economic sanctions on Moscow if the separatists do not leave the premises seized during the last two weeks.
More sanctions
On Monday, a spokeswoman for US Secretary of State John Kerry said: "If they don't take steps in the coming days, there'll be consequences.
But Russia hit back, with Foreign Minister Sergei Lavrov calling for the US to "pressure Kiev to stop hotheads from provoking a bloody conflict and to encourage the Ukrainian authorities to strictly fulfil their obligations".
Visa bans and asset freezes on key Russian figures have already been imposed after Moscow's action in Ukraine last month.
On Easter Sunday, at least three people were killed at a checkpoint manned by armed separatists, with Russia blaming Ukrainian nationalist group Right Sector.
The Organisation for Security and Co-operation in Europe (OSCE) has deployed around 100 mediators in Ukraine to monitor the situation in the east of the country.
