French Foreign Minister Laurent Fabius warned Tuesday that sanctions against Russia could come as early as this week if Moscow does not respond to Western proposals to solve the crisis in Ukraine.

"If they respond positively (to the proposals), (U.S. Secretary of State) John Kerry will go to Moscow, and then sanctions will not be immediate. If they do not respond or if they respond negatively, there will be a series of sanctions that could be taken as early as this week," he said on France Inter radio.

The importance of language in the escalating crisis in Ukraine came to the fore when Russian President Vladimir Putin justified deploying troops in Crimea by saying Moscow needed to protect Russian-speakers there.

Traditionally, the west of the country as well as the capital Kiev has been Ukrainian-speaking, while the east and south -- closer to Russia and including the explosive peninsula of Crimea -- speak Russian.

Russia has refused to budge from its seemingly imminent annexation of Crimea, defying Western pressure at the U.N. Security Council, as Ukraine's ousted president prepared to make his first public appearance in more than a week Tuesday.

As Kremlin-backed forces tightened their grip on Crimea, Russia rebuffed pressure from Western members of the Security Council on Monday to change course on a secession referendum in the strategic peninsula.

U.S. officials including FBI agents are in Kiev helping a Ukrainian-led investigation into corruption under ousted president Viktor Yanukovych, the U.S. ambassador to Kiev said on Monday.

"We are doing what we can to unpack the enormous kleptocracy which surrounded the Yanukovych government," Geoffrey Pyatt said at a press conference in Kiev.

U.S. President Barack Obama spoke late Sunday with Chinese President Xi Jinping about ways to peacefully resolve a tense crisis over control of Ukraine's Crimea region, the White House said.

"They affirmed their shared interest in reducing tensions and identifying a peaceful resolution to the dispute between Russia and Ukraine," the White House said in a statement released on Monday.

The ousted president of Ukraine, Viktor Yanukovych, is to speak in southern Russia on Tuesday in his first public statement in over a week, Russian news agencies reported.

"The statement will be made in Rostov-on-Don. The exact time and place will be announced later," the agencies cited an unnamed source in his entourage as saying Monday.

A journalist eyewitness and a family member said four Ukrainian activists were missing after being led away by pro-Russian militants in two separate incidents in and around Crimea Sunday.

Oleksy Byk from the Internet journal Glavkom told Agence France Presse he saw some 20 gunmen around two women kneeling on the ground with their arms tied near the village of Armyansk on the unofficial border between Crimea and the rest of Ukraine.

The United States is sending a dozen F-16 fighter jets to Poland as a part of a training exercise, amid continuing tensions between Ukraine and Russia, the Polish defense ministry said on Sunday.

Three hundred U.S. service personnel will also be sent to Poland as part of the exercise. The deployment will be completed by Thursday.

Ukraine said Sunday it could sign later this month part of a crucial EU agreement for greater integration, even as the former Soviet state remains in a state of upheaval.

"The political association with the European Union could be signed on March 17 or 21," Foreign Minister Andriy Deshchytsya said in an interview with Ukrainian 1+1 television.

Pro-Russian activists attacked a pro-Kiev rally in Crimea with clubs and whips on Sunday as thousands took to the streets across Ukraine in rival demonstrations, escalating separatist tensions in the troubled ex-Soviet state.

Interim Prime Minister Arseniy Yatsenyuk vowed Ukraine would not cede "an inch" of its territory to Moscow after Russian forces and pro-Kremlin gunmen took over the Black Sea peninsula.