MINSK, Belarus — Ukraine and the Russian-backed rebels have signed a cease-fire deal, a European official at the talks said today.

The announcement by OSCE’s Heidi Tagliavini came as representatives of Ukraine, Russia, pro-Russian rebels and the Organization for Security and Cooperation in Europe met in the Belarusian capital of Minsk.

Since mid-April, Moscow-backed separatists have been fighting government troops in eastern Ukraine in a conflict the U.N. estimates has killed nearly 2,600 people. On Thursday, a NATO military officer told The Associated Press that the ranks of Russian soldiers directly involved in the conflict have grown.

Earlier today, Associated Press reporters heard heavy shelling north and east of Mariupol. The key southeastern port of about 500,000 lies on the Sea of Azov, between Russia to the east and the Crimean Peninsula to the west, which Russia annexed in March. The shelling appeared to indicate that rebels had partially surrounded the area and were probing its defenses.

The seizure of Mariupol would give the rebels a strong foothold on the Sea of Azov and raise the threat that they carve out a land corridor between Russia and Crimea. If that happens, Ukraine would lose another huge chunk of its coast and access to the rich hydrocarbon resources the Sea of Azov is believed to hold. Ukraine ready lost about half its coastline, several major ports and untold billions in Black Sea mineral rights with Russia’s annexation of Crimea.

“Mariupol is a strategic point. If we lose it then we could lose the entire coastline, the whole south of Ukraine,” said Tatyana Chronovil, a prominent Ukrainian activist at a mustering point for the volunteer Azov Battalion on the eastern edge of the city.

The rebel offensive follows two weeks of gains that have turned the tide of the war against Ukrainian forces, who until recently had appeared close to crushing the five-month rebellion in the east.