The Pentagon said late Friday that Russian aircraft had entered Ukrainian airspace “on several occasions in the last 24 hours” and called on Moscow to take “immediate steps” to de-escalate rising tensions.

The air incursions, which officials said may have been part of an effort to test Ukraine’s radar, took place as tens of thousands of Russian troops, massed on Ukraine’s southern and eastern borders for weeks, began “military drills” announced Thursday.

As skirmishing continued in eastern Ukraine between government forces and pro-Russia militants, the halting U.S.-European effort to impose additional sanctions on Russia picked up speed amid a flurry of calls among top government leaders and agreement that last week’s deal with Moscow to calm the situation was all but dead.

European Union foreign ministers planned to meet Monday in Brussels to approve new asset freezes and visa bans on at least 15 prominent Russians close to President Vladi­mir Putin and deemed responsible for the deteriorating situation in Ukraine, a European official said.

The United States, which has its own target list, held off announcing new sanctions as President Obama called the leaders of Britain, France, Germany and Italy to confirm they were on the same page.

“We will have to act,” German Chancellor Angela Merkel said, “and I think this will be a common European action and a joint action of the G7 states.” The Group of 7 leading world economies also includes Canada and Japan.

In Moscow, officials steeled themselves against possible further sanctions, saying that their economy could handle anything the West threw at it. They continued to warn that they reserved the right to send troops into Ukrainian territory as a matter of “self-defense” if they felt Russian interests were being threatened, comparing the situation to Georgia in 2008, when Russia invaded to defend pro-Russia separatists in a breakaway province.

“There are relevant provisions of the U.N. Charter,” said Russia’s envoy to the United Nations, Vitaly Churkin, in an interview Friday on a Russian news program.

Even before new sanctions hit, Russian markets were in turmoil after the Standard & Poor’s rating agency cut the nation’s credit rating to one notch above junk, citing the political situation. The Russian Central Bank hiked its benchmark interest rate by half a percentage point to 7.5 percent to combat rising inflation, and the ruble, which has plummeted 8.9 percent against the dollar this year, sank further.

Russian Foreign Minister Sergei Lavrov told a forum of young diplomats on Friday that the Ukrainian government needed to halt its attempts to dislodge the separatists who have taken over several cities in the country’s east, saying only then would those militants lay down their arms.

President Obama, who has commented on Ukraine at virtually every stop during a week-long tour of Asia, said that “additional targeted sanctions” against Russia are “ready to go.”

At a news conference in Seoul with South Korean President Park Geun-hye, Obama said the new measures would be imposed “assuming we don’t see any drastic changes in behavior on the part of the Russians.” In addition to more “targeted” sanctions, he said the groundwork was being laid “so that if and when we see even greater escalation, perhaps even military incursion by Russia into Ukraine, that we’re prepared for the sort of sectoral sanctions that would have even larger consequences.”

Europe has thus far resisted sanctions on doing business with wholesale sectors of the Russian economy, which are likely to have a far more negative effect on their own economies than on that of the United States. But the European official, who spoke on condition of anonymity because he was not authorized to publicly discuss the matter, said that European leaders had now agreed to begin planning for sectoral sanctions.

Birnbaum reported from Moscow.