Pro-Russian separatists in eastern Ukraine have detained a team of international military observers.
The separatists seized a bus carrying at least 13 people from the Vienna-based Organization for the Security and Cooperation in Europe (OSCE) near the town of Slovyansk.
The town's self-declared mayor said the group was detained because a Ukrainian military official was traveling with them.
"The sign OSCE does not mean protection for an officer of the (army) General Headquarters. We found an employee of the army headquarters. After an investigation we will decide what we are to do."
The OSCE wrote on Twitter that it had lost contact with the German-led monitoring team.
A U.S. State Department spokeswoman condemned the detentions, calling the tactic repressive and cowardly.
"We are deeply concerned about reports that unidentified gunmen have abducted a Vienna document inspection team - that's of course part of the OSCE - and their Ukrainian escorts in the town of Slovyansk. The team was reportedly taken to the administrative building which is being held by armed pro-Russian separatists."
Pro-Russian militants are occupying government buildings in around a dozen cities in eastern Ukraine.
Earlier Friday, Ukrainian Prime Minister Arseniy Yatsenyuk accused Russia of wanting to occupy Ukraine "militarily and politically," as both Kyiv and Moscow mass troops close to their mutual border. Mr. Yatsenyuk told an interim Cabinet meeting that Moscow "wants to start World War Three."
Meanwhile, U.S. President Barack Obama discussed the Ukraine crisis Friday in a telephone call with French President François Hollande, German Chancellor Angela Merkel, Italian Prime Minister Matteo Renzi and British Prime Minister David Cameron.
According to the White House, Mr. Obama said the United States is prepared to impose targeted sanctions against Russia.
Speaking to reporters in Seoul earlier Friday, Mr. Obama criticized what he called Russia's "further meddling" in eastern Ukraine, saying Russian President Vladimir Putin must decide whether he wants to see his country's already fragile economy weakened further because he failed to act diplomatically in Ukraine.
Russia has reportedly dramatically increased the number of troops deployed along its border with Ukraine. A Ukrainian diplomat at the United Nations told VOA that Moscow has doubled its military presence on the border to about 80,000 troops.
Lavrov, for his part, blamed the West for raising tensions, saying Friday that the pro-Russian militants would only lay down their weapons if the Ukrainian government first clears out its own protesters in the capital.
Underscoring the effect that wider sanctions could have on Russia's economy, credit agency Standard and Poor's cut Russia's credit rating to BBB- . The agency said it is concerned about increased capital outflows from Russia, and said the rating could be cut further if sanctions are tightened.
Washington has accused Moscow of failing to uphold the four-party deal it signed last week calling for all parties in Ukraine to lay down their weapons and vacate public buildings. U.S. Secretary of State John Kerry said Moscow has not taken "a single step" to de-escalate tensions since the deal was signed in Geneva.