By Aleksandar Vasovic and Gleb Garanich
SLAVIANSK, Ukraine (Reuters) - Ukrainian forces said they had killed several pro-Russian militants in clashes as they closed in on the separatist-held city of Slaviansk on Thursday, seizing rebel checkpoints and setting up roadblocks as helicopters circled overhead.
Reuters journalists saw a Ukrainian detachment with five armored personnel carriers take over a checkpoint on a road north of the city in late morning after it was abandoned by separatists who set tires alight to cover their retreat.
However, two hours later the troops pulled back. Local militia moved in and began reinforcing the position with sandbags. It was unclear why the military had withdrawn.
Earlier, the Ukrainian Interior Ministry said its forces and troops from the Defence Ministry had killed "up to five terrorists" while destroying three militant checkpoints northeast of the center.
A separatist spokeswoman in Slaviansk said early in the morning that two fighters had been killed in a clash in the same area, near a road leading to the town of Sviatogorsk.
Ukraine's government said it took back Sviatogorsk from gunmen on Wednesday as it relaunched an "anti-terrorist" operation to regain control of the Russian-speaking east.
It was unclear if Ukrainian troops were preparing to risk storming Slaviansk, a city of 130,000 that has become the military stronghold of a movement seeking annexation by Moscow of the Russian-speaking, industrial regions of eastern Ukraine.
For the past few weeks, they have held back from combat, saying they were concerned about civilian casualties - and about provoking reprisals from Russia, which does not recognize the new Ukrainian government and has warned that it would move to protect Russian-speakers in the east, as it did in Crimea.
As the offensive at Slaviansk was confirmed, Russian President Vladimir Putin said it would be a very serious crime for Ukraine to use its army in the east and said there would be "consequences".
The troops could be planning to impose a cordon around the city to disrupt militants' contacts with the rest of the region and to promote surrender negotiations that have been begun by European OSCE monitors following an international accord signed by Russia and Ukraine's Western allies a week ago in Geneva.
Slaviansk separatist mayor Vyacheslav Ponomaryov was quoted by local online site slavgorod.com.ua saying: "If they break into the city, we are ready for them." His men have machineguns, grenade-launchers and at least one self-propelled artillery gun.
The soldiers seen taking the checkpoint near the village of Khrestyshche had arrived from the direction of Sviatogorsk. They appeared disciplined and well-armed, wearing camouflage with black body armor and black helmets with riot visors.
To the south of Slaviansk, Reuters saw dozens of Ukrainian airborne troops with armored assault vehicles digging in mid-morning and manning a checkpoint on the main road to the regional capital, Donetsk.
At Artemivsk, on the same road, the Defence Ministry said troops repelled an overnight attack on their base by a group of about 100 militants using machineguns and grenades. One soldier was wounded, it added. The Interior Ministry said the raid was led by Russian troops. Moscow denies having forces in Ukraine.
(Additional reporting by Alexander Reshetnikov in Slaviansk and Alastair Macdonald in Kiev; Writing by Alastair Macdonald; Editing by Philippa Fletcher)