Ukraine's richest man pleaded with the government on Monday not to bomb Donetsk, a city of a million people where hundreds of heavily armed pro-Russian rebels have vowed to make a stand after losing control of their bastion in the town of Slaviansk.

The Kiev government has said it will act quickly to seize back more territory from rebels after re-taking Slaviansk in what President Petro Poroshenko called a turning point in the three-month conflict against pro-Russian fighters in the east.

Rebels retreating from Slaviansk, some driving armored vehicles flying Russian flags, poured into Donetsk, 110 km (65 miles) to the north over the weekend. About 1,000 of them held a bellicose rally in the central square on Sunday.

Their commander, a Muscovite using the name Igor Strelkov, was quoted by Interfax news agency as saying his men would fight for the city, which was "much easier to defend than little Slaviansk".

Businesses have closed down and thousands of residents are believed to have fled Donetsk.

The city is the headquarters for Rinat Akhmetov, Ukraine's wealthiest man by far, whose fortune, founded on coal and steel, is put at more than $11 billion by Forbes. He said government forces should show restraint in Donetsk and the surrounding Donbass region.

"Donetsk must not be bombed. Donbass must not be bombed. Cities, towns and infrastructure must not be destroyed," he told Ukraina television. "We must avoid suffering and deaths of peaceful people."

The government said it had carried out an air strike against rebel fighters who had attacked the airport in Luhansk, another eastern city, on Sunday. It accused separatists in the area of opening fire in populated areas under the guise of being government forces.

"They use flags of military paratroopers, Ukrainianian state flags and thus fool the population," military spokesman Oleksiy Dmytrashkivsky said on Monday.

The Defense Ministry said early on Monday separatists had launched 10 attacks on government posts and army positions in the previous 24 hours with mortars and small arms. It gave no details of the incidents or casualties, but said troops had returned fire: "The terrorists were given a fitting reply."

More than 200 Ukrainian troops have been killed as well as hundreds of civilians and rebels in the conflict.

POLITICAL UPHEAVAL

Separatist rebellions erupted in mainly Russian-speaking eastern regions of Ukraine in April after Moscow annexed the Crimea peninsula in March following the overthrow of a pro-Russian president in Kiev in February.

Rebels have been barricaded into government buildings in Donetsk, which they declared capital of an independent "people's republic", but Slaviansk, with 120,000 people, was the town where they exercised the most control.

Strelkov, the self-styled rebel defense minister whose real name is Igor Girkin, was quoted by Interfax news agency as saying 80-90 percent of his men had escaped from Slaviansk. They were now organizing the active defense of Donetsk.

Recapturing Slaviansk has given Ukrainian forces by far their clearest victory after months of patchy performance against the heavily-armed fighters, many of whom are Russians who crossed the border, although Moscow denies supporting them.

Poroshenko ordered the assault on Slaviansk after abandoning a unilateral ceasefire last week, arguing that the rebels had refused to abide by the truce.

"My order is now in effect - tighten the ring around the terrorists," Poroshenko tweeted on Sunday. "Continue the operation to liberate Donetsk and Luhansk regions," he said.

He said the victory in Slaviansk marked the beginning of a turning point in the conflict, though he cautioned that rebels would now regroup and "further tests" lay ahead.

In Slaviansk, around 200 residents lined up in the city's central square on Sunday for meat, potatoes, onions and bread distributed by troops.

"Everything is different now. Tonight is the first night with no shelling," said Mikhail Martynenko, 58, a guard at a local market near Slaviansk. "People are in a better mood and there are more people on the streets. Everyone was afraid. They had no idea when another mortar would come flying."

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