Despite a ceasefire that was brokered between the Ukrainian government and pro-Russian separatists in the east of the country less than two days earlier, fighting resumed on Sunday on the northern outskirts of the city of Donetsk and near the port of Mariupol on the Sea of Azov.

A Reuters reporter saw plumes of black smoke filling the sky near Donetsk’s airport which had been in the hands of government forces. Mortar blasts were heard within the city confines during the night, damaging a bridge where rebels had erected a roadblock.

Donetsk is one of the rebel strongholds in the Russophone east of Ukraine where two breakaway republics have requested annexation by Russia.

Artillery fire was also reported in Mariupol where the rebels opened a new front late last month with the backing of military equipment and soldiers from Russia.

Russia still denies it supports the uprising although satellite images released by NATO showed Russian artillery units crossing the Ukrainian border less than two weeks ago while Russian soldiers were apprehended in the former Soviet republic and later freed.

If the separatists conquer Mariupol, they could establish an overland connection from Russia to the Black Sea peninsula of Crimea it took from Ukraine in March.

Before Russia intervened directly in the conflict, the insurrection had been driven back by the Ukrainian army into the cities of Donetsk and Luhansk and seemed on the verge of collapse.

Both sides accused each other of violating the ceasefire, hours after Russian president Vladimir Putin and Ukraine’s Petro Poroshenko had agreed in a phone call that the truce was holding.

Poroshenko spent Thursday and Friday at a NATO summit in Wales where Western leaders urged Putin to withdraw his forces from Ukraine.

At the summit, NATO also announced plans to expand its defenses in Eastern Europe in response to the Ukraine crisis.

According to NATO and Ukraine, Russia has supplied the separatists with weapons for months, including missile launchers that were used to shoot down a commercial airliner in June, killing nearly three hundred passengers and crew.