Russia on Monday extended by three months the detention of a Ukrainian film director arrested on charges of plotting "terrorist attacks" in Crimea after the peninsula's takeover by Moscow.

"The detention of Oleg Sentsov has been extended until October 11," a spokeswoman for a Moscow court, Yulia Sotnikova, told Agence France Presse.

Retreating pro-Russian insurgents dug in on Monday in Ukraine's sprawling industrial hub of Donetsk after government forces scored a string of morale-boosting victories in the bloody battle for the future of the ex-Soviet state.

The eastern home of one million mostly Russian speakers has been flooded with convoys carrying hundreds of fighters and scores of anti-aircraft guns from five smaller surrounding cities where Ukrainian flags were flying for the first time in three months.

German Foreign Minister Frank-Walter Steinmeier warned Monday that there could be no military solution to the violence in Ukraine and urged progress toward a negotiated settlement of the conflict.

"Even if the situation in eastern Ukraine has shifted in favour of the Ukrainian security forces, there will be no purely military resolution of the conflict," Steinmeier said on a visit to Mongolia according to a statement from his ministry.

Resurgent Ukrainian forces on Sunday pursued retreating pro-Russian rebels after seizing their symbolic bastion in a morale-boosting win that appeared to dim hopes for a ceasefire in the bloody separatist insurgency.

Western-backed President Petro Poroshenko called the moment when his troops hoisted the Ukrainian flag over the militias' seat of power in Slavyansk "a turning point" in a campaign that has killed nearly 500 people and inflamed East-West ties.

Russia has barred a leading member of Crimea's pro-Kiev Tatar community from entering his home region for five years after annexing the region from Ukraine in March.

Refat Chubarov, the chairman of the Tatar assembly, or Mejlis, told Agence France Presse on Sunday he had been blocked from crossing into the Black Sea peninsula and handed an official document banning him from Russian territory until 2019.

Resurgent government forces on Saturday hoisted the Ukrainian flag over pro-Russian rebels' main stronghold after a devastating shelling assault that leveled much of the city but delivered Kiev its biggest success of the campaign.

The self-proclaimed mayor of Slavyansk confirmed to Agence France Presse that insurgents had abandoned the rustbelt city of 120,000. A local resident said by phone that barricades once manned by the camouflage-clad gunmen stood abandoned since the early morning.

An eastern Ukrainian separatist leader urged Russia on Friday to secure a truce or send in troops to help repel pro-Kiev forces, warning that a key rebel-held city could soon fall.

"If Russia does not secure a truce or protect us, the Russian people who live here will be destroyed," Igor Strelkov, the self-proclaimed defense minister of the People's Republic of Donetsk, told Russia's pro-Kremlin LifeNews.

Moscow on Friday broadened legislation against incitement to separatism, a move which analysts say is aimed at muzzling critics of Russia's annexation of Crimea.

The law outlawing public calls threatening Russia's territorial integrity previously applied to news media.

Ukraine's embattled new leader agreed on Friday to immediate crisis talks with rebel commanders and Russia aimed at stemming bloodshed that has threatened his strategic country's survival and ruptured East-West ties.

Clashes in the economically-vital border regions of Lugansk and Donetsk have picked up with renewed vigor since Ukrainian President Petro Poroshenko tore up a 10-day ceasefire earlier this week.

Russia's President Vladimir Putin congratulated U.S. counterpart Barack Obama on America's Independence Day Friday and expressed hope that bilateral relations would "successfully develop" despite their disagreements.

Putin predicted that relations between Russia and the United States "will successfully develop on a pragmatic and equal foundation despite the difficulties and differences," the Kremlin said.