German Chancellor Angela Merkel says Russia risks "massive" political and economic damage if it does not change course in the Ukraine crisis.
In a speech to the German parliament Thursday, Ms. Merkel said Ukraine's territorial integrity is "not up for discussion."
The West and Russia have been locked in a tense standoff over Russia's military incursion into Ukraine's Crimean peninsula, which has a heavy Russian influence.
Crimeans are scheduled to vote Sunday in a referendum on whether to join Russia.
In Washington Thursday, Secretary of State John Kerry testified at a U.S. Senate hearing, reiterating the U.S. position that the referendum violates the constitution of Ukraine, as well as international law and the charter of the United Nations. He estimated Russia may have some 20,000 troops in Crimea.
"There is no justification, no legality to this referendum that is taking place. It violates international law, it violates the U.N. charter, it violates the constitution of Ukraine, and I don't think anybody can believe that a hastily put together, rushed referendum, taking place under the imprint of 20,000 plus troops and all that has happened, without debate, without opportunity, is a genuine referendum."
He said it is not clear whether Russia is willing to negotiate with Ukraine and the international community to resolve the conflict over Crimea peacefully.
"The question mark is, is Russia prepared to find a way to negotiate with Ukraine, with the Contact Group, with the other countries involved, in order to be able to resolve this in a way that respects their legitimate interests -- and they have legitimate interests -- but respects them in a way that doesn't violate international law and isn't at the butt of a rifle and of massive military imprint."
Kerry and his Russian counterpart, Sergei Lavrov, are set to discuss Ukraine Friday in a face-to-face meeting in London.
U.S. President Barack Obama warned Russia again Wednesday that the West will "apply costs" to Moscow if it continues to interfere in Ukrainian affairs.
Speaking at the White House alongside interim Ukrainian Prime Minister Arseniy Yatsenyuk, Mr. Obama said Washington "completely rejects" Crimea's planned referendum Sunday. He said the vote, "patched together in a few weeks," is a violation of international law.
Mr. Yatsenyuk thanked Washington for its support and said his government is "absolutely ready and willing" for talks with Moscow; but, he said Ukraine will never surrender. He also said his government is preparing to sign an association agreement with the European Union later this month.
Meanwhile, U.S. Republican Senator John McCain was set to lead a bipartisan delegation to Kyiv Thursday. A spokesman described the visit as a show of congressional support for the interim government, "and for the Ukrainian people's aspirations for freedom, democracy and territorial integrity."
McCain and his Senate Democratic colleague, Christopher Murphy, visited Kyiv in December, at the height of anti-government protests that eventually forced pro-Russian President Viktor Yanukovych to flee Ukraine.
On Wednesday, leaders of the Group of Seven leading industrialized nations called on Russia to, in their words, "cease all efforts to change the status of Crimea contrary to Ukrainian law."
The G7 -- which comprises Britain, Canada, France, Germany, Italy, Japan and the United States -- also said it will not recognize the referendum's outcome.