Ukrainian President Petro Poroshenko has ordered the army to raise the national flag in the separatist stronghold of Slaviansk, eastern Ukraine, after hearing reports of military successes there.

Photo: AFP / GENYA SAVILOV

Earlier, Interior Minister Arsen Avakov said a large number of pro-Russian separatists had fled the flashpoint city following sustained fire from Ukrainian forces.

Poroshenko's website said the newly appointed head of the armed forces general staff had told him separatist fighters came under mortar fire as they tried to break through government forces' lines.

The separatists had lost one tank and other armoured vehicles, Reuters reports.

"The President gave the order ... for the state flag to be raised on Slaviansk's town council offices," the website said, referring to one of the main buildings where the rebels had been based.

Plea to Russia

Slaviansk, a city of 130,000, and surrounding villages have been the main focus of Ukraine's military drive to force out fighters loyal to rebel commander Igor Strelkov, from Moscow.

Mr Strelkov on Friday made an impassioned appeal to Russia, warning on a rebel website that without Moscow's help the entire region the rebels lay claim to, known as Novorossiya (New Russia), would fall to Kiev's forces.

"Slaviansk will fall earlier than the rest," he wrote.

Russia's foreign ministry accused Ukraine of endangering innocent civilians in the region.

"We call on the Ukrainian authorities to cease firing on civilian targets and residential areas," it said.

Earlier, Ukraine proposed talks with pro-Russian rebels aimed at reaching a durable ceasefire as fighting in the east killed 13 servicemen.

A Western diplomat expected the talks to be held again in eastern Ukraine's major industrial hub of Donetsk, where the rebels control key points.

In some of the worst violence since Mr Poroshenko called off a 10-day unilateral ceasefire on June 30, Ukraine's anti-terrorist operation (ATO) headquarters said at least 13 Ukrainian military personnel were killed on Friday.

In Mr Poroshenko's statement he said he had informed European Union foreign policy chief Catherine Ashton about the offer of talks.

However, separatist officials have suggested the venue could be a problem since rebel leaders could be subject to arrest by Ukrainian authorities if they move out of their strongholds.

"(The talks) cannot be held outside Ukraine because it is felt this would inflate the status of the rebels," the Western diplomatic source said.

More than 200 people on the government side have been killed, as well as hundreds of civilians and rebels, in more than two months of fighting in Russian-speaking eastern regions that want to quit Ukraine and join Russia.