In an emailed statement, Vice said it “is aware of the situation and is in contact with the United States State Department and other appropriate government authorities to secure the safety and security of our friend and colleague, Simon Ostrovsky.”

Mr. Biden, echoing the view of the Ukrainian authorities that the unrest in the east has been instigated and, in some places, directly assisted by Russian military and intelligence personnel, called on Russia to “stop supporting men hiding behind masks in unmarked uniforms,” the so-called green men who have seized government buildings in at least 10 towns and cities.

“It’s time for Russia to stop talking and start acting — act on the commitments they made” in Geneva, Mr. Biden said, adding that Ukraine, through an amnesty law and other steps, was trying to live up to its commitment.

Russia, however, blames Kiev for the slim results of the Geneva agreement, which called for the disarming of gunmen and the freeing of occupied buildings. While Washington and Kiev focus on pro-Russian militants holding buildings in the east, Moscow insists that the main reason for the continuing unrest is the Kiev government’s failure to rein in groups like Right Sector, which are still occupying City Hall and the central post office in Kiev.

Since the ouster in February of Ukraine’s pro-Moscow president, Viktor F. Yanukovych, Russia has repeatedly denounced Ukraine’s new leadership as dominated by extreme nationalists and neo-Nazis who threaten not only ethnic Russians and Russian speakers in the east but Jews and other minorities across the country. After meeting in Kiev with Ukrainian officials and local Jewish leaders, however, David Harris, executive director of the American Jewish Committee, said on Tuesday that “the government is committed in word and, we believe, in deed to fighting xenophobia and anti-Semitism.”

Russian allegations of anti-Semites on the rampage in Ukraine, Mr. Harris said in an interview, were “a dangerous, Machiavellian game” that only endangered Jews. “This is not the first time in history that the Jewish community has been put in the middle of such a game,” he said.