A separatist leader in eastern Ukraine warned Kiev that pro-Russian rebels would take the offensive again after regrouping forces in the region's biggest industrial hub of Donetsk.
"We're not preparing ourselves for a siege, we're preparing ourselves for action," Alexander Borodai said in an interview in Moscow with Russian Internet outlet Gazeta.ru in Moscow.
Militants fled the rebel stronghold of Slaviansk over the weekend, following months of fighting, in a victory for Kiev that President Petro Poroshenko called a "turning point" in its fight against the separatists.
But Borodai, who has been appointed by the rebels as prime minister of their self-proclaimed Donetsk People's Republic, said the escape from Slaviansk by self-styled rebel defence minister Igor Strelkov, a Muscovite, had given the separatists a chance to concentrate their fighting power.
They have at their disposal tanks, armoured vehicles and rocket systems.
"Strelkov is the chief commanding officer. Now a strict vertical of command will be built through all armed units, which I coordinated with greater or less success. Igor and I will be able to build a very effective, clear vertical," he said.
Armed groups have flooded the city of Donetsk since Slaviansk's fall. Two bridges were destroyed on Monday after Ukraine's deputy security council chief said forces would blockade the capitals of the two separatist regions, Donetsk and Luhansk.
"Any blockade of these two cities by the Ukrainian army is impossible. The Ukrainian army and its resources are not in a state to carry out a real blockade on even one Donetsk, so I don't see a real threat in that in the future," he said.
Borodai said he was in consultations with Moscow on the conflict but said he had no insight into the Kremlin's plans and denied they had any direct funding from Moscow.
"Moscow is carrying out consultations with us regarding the regulation of the conflict, and that's why I've come here. But Moscow is not a side in the conflict, does not participate in it," he said.
"There are no direct tranches (of cash) ... But I definitely don't see in the Kremlin's plans a desire to preserve instability in so-called southeastern Ukraine."
Earlier, President Petro Poroshenko named a new chief of military operations against separatists in eastern Ukraine, continuing a shake-up of the military and security leadership to combat the insurgency.
In a decree, he named Vasyl Grytsak head of the "anti-terrorist centre", making him operational chief in the drive to quash the rebels.
Pro-Russian separatists have been fighting government forces since April when they set up 'people's republics' in the east of Ukraine.
The move, announced on the president's website, is the latest by Poroshenko to tighten his grip on security operations and restore public confidence in his ability to win the fight against the separatists.
After the appointment of a new defence minister and a new head of the armed forces general staff last week, government forces notched up their biggest victory to date against the separatists by re-taking the city of Slaviansk which had been a rebel stronghold.
Grytsak, a 20-year veteran of the state security apparatus, replaces Vasyl Krutov who has headed the "anti-terrorist centre" since mid-April.
Despite some other military successes, Krutov and other security officials have come under public criticism for big military losses including the downing by the rebels of an Il-76 cargo plane in June with the deaths of more than 49 crew and servicemen.
Gazprom:
Russia's top gas producer Gazprom said on Tuesday Ukraine had not met its payment deadline for June resulting in its total debt to the company rising to $5.3 billion.
"The unwillingness of Ukraine ... to pay for Russian gas is becoming chronic and once again shows that the move to pre-payment, which is foreseen in the contract, is the only real solution," chief executive Alexei Miller said in a statement.
Earlier, Russia's Gazprom said Russian natural gas flows to Europe via Ukraine remained stable.
Gazprom turned off gas supplies to Kiev last month after Ukraine failed to meet a deadline to pay off some of its gas debts. Europe receives around a third of its gas needs from Russia.
REUTERS