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.

Russia signaled Friday that a deal could be clinched in Iran nuclear negotiations, as there is "political will" and a sense of urgency among participants for an accord before a July 20 deadline.

"One feels political will of the participants, and a certain fear that we may not be quick enough -- that is a good sign in this situation. There is not much time left. But there are chances," Russia's deputy foreign minister Sergei Ryabkov told Kommersant newspaper.

Russian President Vladimir Putin has called for more patriotic education for his country's youth in order to protect them from foreign influences in what he views as a global ideological battle.

"There is a tough battle in the world for hearts and minds, for ideological and informational influence," Putin said at a government meeting on Thursday, cautioning against "artificial conflicts" stirred by foreign states, including among different ethnicities.

Russian film director Fyodor Bondarchuk has been chosen for the big-budget Hollywood epic Odysseus, one of the film's producers told Agence France Presse Thursday.

"Warner Bros chose Fyodor to direct Odysseus," producer Michael Schlicht said. "It will be a huge spectacular epic."

Russian President Vladimir Putin on Thursday expressed "deep concern" over the growing number of civilian casualties and refugees in eastern Ukraine.

"Vladimir Putin expressed deep concern over the growing number of peaceful civilians and a sharp increase in the number of refugees from southeastern Ukraine in Russia," the Kremlin said in a statement after a telephone conversation between Putin, German Chancellor Angela Merkel and French President Francois Hollande.

Fighting rumbled on in east Ukraine Thursday between government troops and pro-Russian rebels despite a fresh push for ceasefire talks from international envoys. 

Ukraine's border service said that nine guards were wounded when rebels shelled the Dolzhansky border post with Russia, a strategic crossing that Kiev won back days earlier in a step hailed as the "first victory" since the renewal of its military offensive.