This is one of the images provided to the OSCE by the Ukrainian government. The red circle shows a man they have identified as a Russian soldier in Slovyansk on April 14. (Obtained by The Washington Post)

The United States and Russia traded charges Monday over ­implementation of last week’s agreement to defuse tensions in Ukraine, with Secretary of State John F. Kerry urging Moscow to publicly call on armed pro-Russian separatists to vacate occupied government buildings in eastern Ukrainian cities and to send a senior diplomat to tell them Russia wants them to leave.

Kerry’s comments, in a telephone call to Russian Foreign Minister Sergei Lavrov, came after separatists blocked international monitors from the Organization for Security and Cooperation in Europe from the occupied buildings. In some cases, the monitors were stopped at separatist checkpoints outside urban areas, according to Western diplomats.

An OSCE team “has been in Donetsk but has not yet made progress,” one diplomat said. “They are trying to get back into Slovyansk, but pro-Russian demonstrators have been blocking the way.” An OSCE assessment released Monday described the situation as “very tense” in Donetsk and as “deteriorating” in Slovyansk, where “the entire town is under the control of armed groups.”

Free passage of the monitors and the disarming of separatists were key elements of the agreement reached in Geneva on Thursday among the United States, Russia, Ukraine and the European Union, and the very terms that U.S. officials said would be the primary test of whether Russia intended to comply with the accord.

U.S. officials said late last week that they expected to see results by Monday. Collapse of the agreement — which Lavrov on Monday accused Ukraine of violating — would almost certainly lead to additional U.S. sanctions against Russian individuals.

But a U.S. response is unlikely until at least after Vice President Biden returns home Wednesday from a visit to the Ukrainian capital, Kiev, according to a senior State Department official speaking on the condition of anonymity to discuss internal policy deliberations.

Although there are already “several lists” of Russians drawn up for new sanctions, the senior official noted that the president and others will be traveling for much of the week. President Obama leaves Tuesday for Asia, before Biden’s return and just as Kerry ends an Easter holiday in Boston.

Officials indicated that the administration is not yet ready to impose “sectoral” measures against major parts of the Russian economy, which Obama authorized early this month in the event of a Russian military incursion into Ukraine.

In a news conference Monday, Lavrov appeared to threaten just that, blaming Ukraine’s interim government for sparking the violence and saying that Russia might be forced to step in if it does not stop. About 40,000 Russian troops are massed on Ukraine’s southern and eastern borders.

Last week, the Kiev government, with U.S. backing, provided photographs to the OSCE that it said proved the presence of “Russian sabotage-reconnaissance groups” already in eastern Ukraine and directing the seizures in the eastern cities of Slovyansk and Kramatorsk.

State Department spokesman Jen Psaki said “we feel confident” that the photographs constitute “further evidence” of U.S. contentions that Russia has armed and organized the separatists.

Asked whether the U.S. assessment is bolstered by other information that is not publicly available, Psaki said, “We’re looking at a fair share of classified and unclassified information,” including the photos, most of which are publicly available on social media.

The photographs include a bearded man whose uniform bears a Russian special forces insignia. He was pictured among armed separatists in Kramatorsk and Slovyansk last week. Another photo shows the same person in Georgia during a 2008 incursion by Russian troops.

The Ukraine Security Service put out a wanted poster for a man it identified as Ihor Ivanovych Strielkov, a Russian intelligence officer. The poster accused him of a series of charges, included premeditated murder and organizing mass riots.

The Ukrainians allege that Russian agents have been traveling among cities and towns in the eastern part of the country this month, organizing and arming separatists, similar to Russian activities prior to last month’s annexation of the Ukrainian region of Crimea.

In a report issued Monday evening, the OSCE described being approached by many eastern Ukrainians who expressed a desire to be part of Russia and discounted the Geneva agreement.

Under the accord, the United States and Russia each committed to appointing representatives to the monitoring team. It was unclear how many Russians are on the teams that traveled through the eastern part of the country over the weekend and on Monday.

In his talk with Lavrov, Psaki said, Kerry “called on Russia to assign a senior diplomat to work with the OSCE mission . . . to make absolutely clear to the separatists that Russia supports the agreement and wants de-

Although some of Obama’s senior national security advisers, particularly at the State Department and the Pentagon, have urged a move to sanctions against Russian economic sectors, including energy, mining and defense, the administration plans to hold them in reserve. Instead, barring a major Russian military move, it is likely to add to its list of Russian officials and what it calls “cronies” of President Vladi­mir Putin who are already subject to asset freezes and visa bans.

“You always have these debates,” said Howard Mendelsohn, who served as the administration’s deputy assistant secretary for the Treasury Department’s Office of Intelligence and Analysis.

But even as the White House wants to be “very, very targeted and very, very nuanced in terms of rolling out these measures,” even limited sanctions on prominent individuals can trigger broader impacts on a national economy, Mendelsohn said.

“The reality of financial institutions is that they’re [already] very much in pause mode in terms of any Russia-related risk,” he said. Although “three months ago, no one really would have asked very much about Russia,” that situation has changed.

“Now the examiners, who are going to be the first ones whose heads are rolling if there’s a mistake,” are concerned not only about current sanctions, but also with the overall environment for Russia-related risks. “In the end, it creates a very sort of stifling environment” for doing business with Russia, Mendelsohn said.