KIEV, Ukraine — Russia agreed to international monitors arriving in Ukraine after Western nations voiced growing concern over the Kremlin massing troops on the border with its neighbor.

While talks about the monitors with Russia were “difficult,” their presence may help avoid escalation, German Foreign Minister Frank-Walter Steinmeier told reporters in Kiev Saturday. The mission will include 100 monitors, which may increase to 400 across Ukraine, the Vienna-based Organization for Security and Cooperation in Europe said Friday.

The announcement came even as Russian troops stormed a Ukrainian airbase in Crimea and issued an ultimatum to forces inside to surrender, the deputy commander of the base in Belbek, near Sevastopol, said on Saturday.

An Associated Press journalist reported that shots were fired at a Ukrainian air force base in Crimea that has been blocked off by Russian forces.

A webcam from the base showed two armored vehicles at the base's gate and an ambulance entering the grounds. Russian forces have been seizing Ukrainian military facilities for several days in the Black Sea peninsula, which voted a week ago to secede and join Russia.

The commander of Belbek airbase in Crimea said told Reuters news service he would be taken away by Russian forces for talks. Reuters reported that  one Ukrainian was injured in the takeover of the base, which had been one of the last military facilities in Crimea still under Ukrainian control, Ukraine's Colonel Juliy Mamchur told Reuters.

Asked if he thought he would return safely from the talks at an unspecified location, he said: "That remains to be seen. For now we are placing all our weapons in the base's storage."

The six-month monitoring mission is meant to cool tensions in the worst standoff between Russia and the West since the end of the Cold War. As Russian President Vladimir Putin yesterday completed the annexation of Crimea, which isn’t part of the OSCE’s mandate, the two sides exchanged salvos of sanctions, raising concern about further escalation.

The OSCE “will continue their efforts to rebuild bridges and find cooperative solutions to the major political and security challenges that Europe is now confronted with,” Swiss Foreign Minister Didier Burkhalter, the chairman of the 57- nation group that focuses on conflict prevention and preserving human rights, said in the statement.

With Putin’s annexation of Crimea completed, attention shifted to whether Russia would seek to claim other parts of Ukraine. White House National Security Adviser Susan Rice said the U.S. was monitoring developments on Russia’s frontier with Ukraine.

“The Russians have stated that they are intending military exercises,” Rice said at a briefing yesterday in Washington. “Obviously, given their past practice and the gap between what they have said and what they have done, we are watching it with skepticism.”

The presence of Russian forces near Ukraine’s border and protests by pro-Russian activists in the east and south of Ukraine have raised alarms that Putin may push further into the second-most-populous former Soviet republic. European leaders signaled that Russia may face further repercussions if it doesn’t stop what they see as destabilizing actions.

If Russian troops enter east Ukraine, it “would trigger far-reaching consequences in a broad range of economic areas,” U.K. Prime Minister David Cameron told reporters yesterday after a summit in Brussels. “That must include the key areas like finance, like the military, like energy,” he said, adding that “Russia needs Europe more than Europe needs Russia.”

Ukraine’s government, which yesterday signed the political chapters of an association accord with the EU, discussed military cooperation with European partners, Prime Minister Arseniy Yatsenyuk said in Kiev today.

Russia’s moves to claim Crimea highlight Putin’s disregard for the post-Cold War security order in Europe, according to Andrew Kuchins, director of the Russia and Eurasia Program at the Center for Strategic and International Studies in Washington.

“There’s nothing Vladimir Putin would rather do than to de-legitimize the post-Cold War order, expose the transatlantic partnership as a sham, and deeply degrade U.S. leadership in the world,” Kuchins said at a media briefing yesterday. “He’s already gone pretty far down that path in the past three weeks.”

Leaders of the U.S., the EU, China, Japan and other nations meet in The Hague starting on March 24, and President Barack Obama plans to use the gathering to mobilize opposition to Russia’s incursion into Crimea.

While ruling out military action, Obama has joined European leaders in warning of further consequences if Russia fails to yield. The U.S. is focusing on diplomatic and economic tools to defuse the crisis, Rice told reporters.

“Our interest is not to see the situation escalate and devolve into hot conflict,” she said.

The EU, moving more slowly than the U.S. on sanctions, yesterday expanded to 51 individuals its list of Russians and Ukrainians punished with asset freezes and travel bans.

Twelve new names published by the EU include five officials close to Putin who already face U.S. sanctions. Among them is Russian Deputy Prime Minister Dmitry Rogozin, who mocked his banishment by the U.S. as the product of “some joker,” and two deputy commanders of the Black Sea fleet in charge of Russian forces that occupied Crimea.

The U.S. on March 20 widened its list of people targeted to 27 Russian officials and four Ukrainians. In addition, Obama that day authorized potential future penalties on Russian industries, including financial services, energy, metals and mining, defense and engineering.

Those targeted by the U.S. include billionaire Gennady Timchenko, a co-founder of oil trader Gunvor Group Ltd., and Arkady Rotenberg, a former judo partner of Putin whose companies won more than $7 billion in contracts for the Winter Olympics.

Russia’s Foreign Ministry is proposing retaliatory steps, as “unanswered sanctions may whet appetites to impose new measures,” Deputy Foreign Minister Grigory Karasin told lawmakers. The country reserves the right to impose sanctions following the EU’s decision to expand its list, Russia’s Foreign Ministry said in a statement Saturday.

The EU said yesterday that Putin so far hasn’t crossed the destabilization threshold. Special trading relationships that several European countries have with Russia, coupled with the fallout from the debt crisis that came close to wrecking the euro, are frustrating a tougher response.

Sanctions require the agreement of all EU governments, a process that can’t match Putin’s speed in mobilizing troops, staging a secession referendum in Crimea and moving to annex the Black Sea peninsula.

Putin says ethnic Russians in the region are at risk from the government in Kiev, a claim that Ukraine denies. Russia backs the recently appointed administration in Crimea that held the disputed March 16 ballot, in which almost 97 percent backed joining Russia.

Acting Ukrainian President Oleksandr Turchynov said his country doesn’t accept Russia’s takeover and won’t allow Russian forces on its mainland. He said yesterday that Ukraine would submit a plan to demilitarize Crimea.

“Ukraine will do everything in order to free the occupied territories,” Turchynov told reporters in Kiev after a meeting with United Nations Secretary General Ban Ki-moon.