KIEV, April 15 - Ukrainian forces launched a "special operation" on Tuesday against separatist militia in the Russian-speaking East, authorities said, although aside from a landing by airborne troops the action was limited, Reuters reported.

Soldiers disembarked from two helicopters at an airfield at Kramatorsk, where reporters earlier heard gunfire that seemed to prevent an air force plane from landing. The troops withdrew into barracks after local civilians manning a barricade gave them a hostile reception when they tried to leave the compound.

Acting President Oleksandr Turchynov declared a much-needed victory over pro-Russian rebels by saying the air base had been "liberated." But there was no sign of militants.

A senior Ukrainian officer told the unarmed crowd that he had come to direct the "anti-terrorist operation" that Turchynov announced earlier in the day, after more than a week of missed deadlines set by Kiev for armed pro-Moscow activists to end occupations of public buildings in some 10 places in the east.

But after a scuffle with some of the hundreds who chanted hostility to the new Ukrainian authorities, some of them holding Russian flags, the troops pulled back at dusk.

Ukraine's state security service said an "anti-terrorist" operation was also in progress against separatists in the nearby town of Slavyansk but there was no immediate evidence of action.

Nonetheless, Kiev's stated resolve to challenge militants it says are orchestrated by the Kremlin, marked an escalation of the deepest East-West crisis since the Cold War.

Russian President Vladimir Putin, who is seen by Western diplomats as intent on reasserting Moscow's influence across what was the Soviet Union and beyond, spoke by phone with German Chancellor Angela Merkel, according to Russia's Itar-Tass news agency, saying, "The sharp escalation of the conflict puts the country, in essence, on the verge of a civil war."

The standoff has raised fears in the West and in Kiev that Russia might intervene militarily to "protect" Russian speakers in eastern Ukraine, following its annexation of the Crimean region last month in response to the overthrow of Moscow-backed president Viktor Yanukovych following months of protests.

The Russian foreign ministry said it was "deeply concerned" by what it said were reports of casualties in eastern Ukraine, though it was unclear where any such incidents had taken place.

A spokesman for U.S. President Barack Obama said Ukraine's government was obliged to respond to "provocations" in the east but Washington was not considering sending arms to Kiev.

It was "seriously considering" adding to sanctions imposed after the annexation of Crimea, the White House said, although the State Department said such action was unlikely before a meeting in Geneva on Thursday at which U.S., EU and Ukrainian officials will try to persuade Russia to defuse the situation.

Turchynov said an offensive he first announced on Sunday was now in progress after days in which it failed to materialize.

"The anti-terrorist operation began during the night in the north of Donetsk region. But it will take place in stages, responsibly, in a considered way," he told parliament. "I stress again: The aim of these operations is to defend the citizens of Ukraine."

At least 15 armored personnel carriers displaying Ukrainian flags were parked by the side of a road around 50 km (30 miles) north of Slavyansk, witnesses said.

Ukrainian troops wearing camouflage gear and armed with automatic weapons and grenade-launchers were stationed nearby, with a helicopter and several buses containing interior ministry personnel near the road.

In Slavyansk itself, separatists have seized the local headquarters of the police and state security service. (rt/ez)