Ukrainian President Petro Poroshenko (L) addresses members of the Ukrainian special task police squadron in the eastern Ukrainian town of Slaviansk, July 8, 2014. REUTERS/Gleb Garanich

By Richard Balmforth and Maria Tsvetkova

KIEV (Reuters) - Ukraine's government kept up military pressure against pro-Russian rebels on Tuesday, threatening them with an "nasty surprise", while the militants said they were preparing to fight back after losing their main stronghold.

President Petro Poroshenko, drawing confidence from the fall of the rebel bastion of Slaviansk at the weekend, named a new chief of military operations in the east following his appointment of an aggressive new defence minister who again demanded the separatists lay down their arms.

A security official said the government's plan to clear rebels from the two big towns of Donetsk and Luhansk would come as an "nasty surprise" for the insurgents.

But Poroshenko - whose officials have ruled out any more unilateral ceasefires - kept the door open to a further round of indirect peace talks with separatist leaders, naming a possible venue in a government-controlled monastery-town in the east.

Poroshenko on Tuesday visited Slaviansk, which lies in eastern Ukraine's industrialised Donbass region.

"Until today Slaviansk was a symbol of terror and violence. Today Slaviansk is a symbol of a free Donbass and I thank you for that," he said on the city's main square in front of what was one of the rebels' main headquarters.

Meanwhile, signs emerged of a split in separatist ranks over the fall of Slaviansk with a powerful field commander critically questioning the pull-out from the rebel stronghold.

The rebels' loss of Slaviansk marks a major breakthrough in Kiev's three-month long fight against Russian backed separatists who are now calling in vain for military help from Moscow.

One rebel leader played down its loss as a military expedient and said the hundreds of fighters who were able to move from the town to the regional capital Donetsk were preparing a command structure to defend that city and hit back:

"We're not preparing ourselves for a siege. We are preparing ourselves for action," Alexander Borodai, prime minister of the self-proclaimed Donetsk People's Republic, told a Russian online newspaper during a visit to the Russian capital.

Sporadic shooting was heard from parts of Donetsk overnight. In Luhansk, a city on the border with Russia where rebels also control key buildings, two people in a minibus were killed by a shell that exploded nearby, a municipal official said.

"There is an exchange of fire among the separatists. They are shooting at each other," Iryna Verigina told a Ukrainian television station by telephone from Luhansk.

Poroshenko, installed in office just a month ago, named Vasyl Grytsak to head the "anti-terrorist centre", making him operational chief in the drive to crush the rebels.

The move continued his shake-up of the military and security leadership in which he has appointed a hardline defence minister to bring fresh vigour to the fight against the insurgency.

Grytsak, a 53-year-old police lieutenant-general and 20-year veteran of the state security apparatus, replaces Vasyl Krutov, who had headed the "anti-terrorist centre" since mid-April.

Despite some successes against the rebels, Krutov and other security officials have come under criticism for the patchy performance of the armed forces and big military losses including the downing by the rebels of an Ilyushin Il-76 plane in June with the deaths of more than 49 crew and servicemen.

FOCUS ON DONETSK

Pro-Russian rebels have been fighting government forces since April when they set up separatist republics in the Russian-speaking east after political upheaval in Kiev led to the ousting of a Moscow-backed president followed by Russia's annexation of Crimea.

They have brought down military helicopters and ambushed government forces on the ground in three months of fighting in which more than 200 Ukrainian troops have been killed, along with hundreds of civilians and rebels.

The fall of Slaviansk to government forces at the weekend has now swung the focus onto Donetsk, raising the question of how the Kiev military will go about breaking the resistance in a sprawling industrial city with a population of over 900,000.

Security officials in Kiev gave away nothing about their military plans. But a spokesman for the "anti-terrorist operation", Andriy Lysenko, said: "There is a plan ... under which we will be able to liberate these towns (Donetsk and Luhansk).

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