KIEV, July 8 - Ukraine's government kept up military pressure against pro-Russian rebels on Tuesday, threatening them with a "nasty surprise," while the militants said they were preparing to fight back after losing their main stronghold, Reuters reported.

President Petro Poroshenko, drawing confidence from the fall of the rebel bastion of Slovyansk at the weekend, named a new chief of military operations in the east following his appointment of an aggressive new defense 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 a "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 Slovyansk, which lies in eastern Ukraine's industrialized Donbas region.

"Until today Slovyansk was a symbol of terror and violence. Today Slovyansk is a symbol of a free Donbas 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 Slovyansk with a powerful field commander critically questioning the pull-out from the rebel stronghold.

The rebels' loss of Slovyansk 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 Hrytsak to head the "anti-terrorist center", 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 defense minister to bring fresh vigor to the fight against the insurgency.

Hrytsak, 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. (rt/ez)