Secretary of State John Kerry warned Thursday that Russia will face immediate, "very serious" steps from the United States and Europe if it annexes Ukraine's Crimea region.

Kerry told a Senate committee that Moscow should expect that the U.S. and the European Union to take measures against it on Monday should it accept and act on the results of this weekend's referendum in Crimea.

Crimeans are to vote on joining Russia on Sunday in a vote that the U.S. and the EU say violates Ukraine's charter and international law.

"If there is no sign of any capacity to be able to move forward and resolve this issue, there will be a very serious series of steps on Monday in Europe and here with respect to the options that are available to us," Kerry warned.

He told lawmakers the U.S. estimated there were currently about 20,000 Russian troops on the Crimea peninsula, below the 25,000 allowed under an agreement with Moscow on stationing its bases there. Amateur photographs near the border showed an increase in military presence ahead of the Sunday referendum, the New York Times reported. The Russian Defense Ministry confirmed that 10,000 soldiers are conducting intensive training exercises in regions along Ukraine's eastern border.

Kerry stressed that for the time being Russia did not "have the assets ... necessary to be able to march in and take over Ukraine," although he acknowledged that could change. Kerry said there are contingencies in place in case Russian forces move farther east into Ukraine. He said he hopes to avoid taking action but added that there’s “no telling that we can.”

Ukrainian interim Prime Minister Arseniy Yatsenyuk said he saw a risk of war as Russia is “ready to invade,” with international efforts standing by to end the “aggression.”

Russian President Vladimir Putin said Moscow did not create the situation Ukraine is facing. “I would like to assure you that Russia was not the initiator of the circumstances we are talking about now," he told reporters at a news conference in Sochi, Russia.

Russia has voiced support for the deployment of a European security monitoring mission in Ukraine, including Crimea, the chairman of the Organization for Security and Cooperation in Europe (OSCE) said Thursday, calling this a possible "big step forward."

"The Russian Federation supported the idea of a rapid approval and rapid deployment of a special monitoring mission for Ukraine," Thomas Greminger, Switzerland's ambassador to the OSCE, told reporters after a meeting of the group's envoys in Vienna.

Kerry meets in London on Friday with Russia's foreign minister, Sergei Lavrov, in a last-ditch bid to avert a crisis over the referendum. Kerry said he hoped that "reason would prevail" but stressed there was no guarantee that it would.

U.S. House Speaker John Boehner on Thursday urged the Senate to pass a House bill backing $1 billion in loan guarantees for Ukraine.

Al Jazeera and wire services