* Tension rises on eve of four-power talks on Ukraine in Geneva
* Government forces tighten control on Kramatorsk
* Rebels drive armoured vehicles into Slaviansk
* Moscow, Kiev trade charges of "civil war" and "terrorism"
By Gabriela Baczynska and Thomas Grove
KRAMATORSK/SLAVIANSK, Ukraine, April 16 (Reuters) - U krainian government forces and separatist pro-Russian militia staged rival shows of force in eastern Ukraine on Wednesday amid escalating rhetoric on the eve of crucial four-power talks in Geneva on the former Soviet country's future.
Government troops drove seven armoured personnel carriers flying the Ukrainian flag into the town of Kramatorsk after securing control of a nearby airfield from the rebels on Tuesday, prompting Russian President Vladimir Putin to warn of the risk of civil war.
But just 15 km (9 miles) away, armed men in different types of combat fatigues drove six armoured personnel carriers, one flying the Russian flag, into the town of Slaviansk, stopping outside the town hall, which is occupied by separatists.
The armed men waved as they drove in, and some people waved back and shouted: "Well done lads!" and "Russia" Russia!"
Overhead, a Ukrainian jet fighter carried out several minutes of aerobatics above the town's main square in a clear show of strength by Kiev's forces.
In the industrial city of Donetsk, at least 20 armed separatists occupied the city council building, a spokeswoman for the council said.
The muscle-flexing and inflamed rhetoric heightened fears of violence after Moscow-backed gunmen occupied public buildings in 10 eastern towns and cities in the last week.
The Kiev government is seeking to reassert control slowly and without bloodshed before Thursday's Geneva meeting at which the Russian and Ukrainian foreign ministers are due to meet for the first time in the presence of the United States and the European Union.
Russia, which has refused to recognise Ukraine's pro-Western government since Moscow-backed President Viktor Yanukovich was ousted by mass protests in February, sought to dramatise instability in its neighbour ahead of those talks.
BRINK OF CIVIL WAR
Putin told German Chancellor Angela Merkel in a telephone call late on Tuesday that Kiev had "embarked on an anti-constitutional course" by using the army against the rebels.
"The sharp escalation of the conflict puts the country, in effect, on the brink of civil war," a Kremlin statement quoted him as saying.
Ukrainian Prime Minister Arseny Yatseniuk reacted by accusing Moscow of "exporting terrorism to Ukraine".
Russian Foreign Minister Sergei Lavrov, speaking in Vietnam before heading to Geneva, said Kiev should listen to what he called the voice of the people of Ukraine and avoid force.
"It is unacceptable to use (the armed) forces in the eastern Ukraine," he told reporters in Hanoi.
The Ukrainian government launched what it called a "special operation" on Tuesday against separatist militia in the Russian-speaking East, although aside from a landing by airborne troops the action was limited and avoided casualties.
Soldiers disembarked from two helicopters at the airfield 10 km (6 miles) from Kramatorsk, where reporters heard gunfire that seemed to prevent an air force plane from landing.
There was no sign of violence in the area on Wednesday, but civilians watching the armoured vehicles enter the town reflected the sharp political divisions in the mainly Russian-speaking southeastern Donbass region.
A group of about 30 local residents blocked the APCs briefly and tried to prevent them going through, a Reuters witness said. Soldiers dismounted and pushed them away. One shot was fired in the air in a brief scuffle before the vehicles moved on.
The protesters managed to take away one hand-held radio and two rifle magazines from soldiers.
"I think Donbass should be an independent country allied with Russia," said a local resident who gave his name as Olexander. "My homeland is the Soviet Union. We just need to chop off the rotten west of Ukraine and we'll be fine."
Elsewhere in Kramatorsk, there was no overt sign of hostility as several hundred people milled around another cluster of six APCs from the 25th paratroop brigade from Dnieperpetrovsk. Some residents gave the soldiers tea and bread.
But one man, who gave his name as Sergei, said the troops were unwelcome, contrasting the use of the army with the authorities' tolerance of a protest camp on the central Maidan square in the capital.
"I don't like these troops. As far as Kiev is concerned, we are not people," he said. "For some reason they didn't send tanks onto the Maidan (Independence Square in Kiev) but they send troops to us. Donbass will not forgive this. The country does not exist any more."
While troops were not deployed during the protests that ousted Yanukovich in Kiev, police snipers shot dead dozens of protesters.
The United States and the EU have accused Moscow of orchestrating the separatist operation in eastern Ukraine as it did in the Ukrainian Black Sea province of Crimea before annexing the region last month.
Russia, which Western governments says has massed about 40,000 troops just across the border with eastern Ukraine, denies the charge. The Kremlin is demanding that Kiev accept a loose federal structure for Ukraine.
On Thursday, acting Ukrainian President Oleksander Turchinov declared a much-needed victory over pro-Russian rebels by saying the Kramatorsk air base had been "liberated". But the government made no immediate attempt to dislodge separatist gunmen elsewhere.
Ukraine's state security service said an "anti-terrorist" operation was also in progress against separatists in the nearby town of Slaviansk 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.
The standoff has raised fears in the West and in Kiev that Russia might intervene militarily to "protect" Russian speakers in eastern Ukraine,
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.
The White House said it was seriously considering adding to sanctions imposed after the annexation of Crimea, but the State Department said such action was unlikely before the Geneva meeting. (Additional reporting by Christian Lowe in Moscow, Richard Balmforth in Kiev and Nguyen Phuong Linh in Hanoi; Writing by Paul Taylor; Editing by Will Waterman and Giles Elgood)