DONETSK, Ukraine (AP) — One civilian was killed in shelling in the Ukrainian port city of Mariupol and large explosions were heard near the airport in Donetsk early Sunday, raising fears that a cease-fire signed two days ago is on the verge of collapse.

Blasts from the area near the airport were powerful enough to be heard in downtown Donetsk, the main rebel-held city in eastern Ukraine.

A spokesman for Ukraine's national security council, Volodymyr Polyovyi, said at a briefing in Kiev that rebels appeared to have tried to attack the airport, which has been under the control of government troops since May and has come under unremitting attacks from pro-Russia separatist rebels since then.

The cease-fire had appeared to be holding for much of the day on Saturday, but shelling started late at night. A rebel statement said Ukrainian forces violated the cease-fire by firing on their positions in six locations on Saturday, including near the Donetsk airport. The statement said several rebels were killed.

Shelling also occurred overnight on the outskirts of the port city of Mariupol, where Ukrainian troops retain defensive lines against the rebels.

The city council there said one civilian was killed and a serviceman was wounded. A shell also destroyed a nearby gas station and the volunteer Azov Battalion said on Facebook that their positions were hit by Grad rockets, but did not give details.

Mariupol is located on the coast of the Sea of Azov, 115 kilometers (70 miles) south of Donetsk. Rebels recently opened a new front on the coast, leading to fears that they were trying to secure a land corridor between Russia and the Crimean Peninsula, which Russia annexed in March.

Ukraine, Russia and the Kremlin-backed separatists signed the cease-fire deal in the Belarusian capita, Minsk, on Friday in an effort to end more than four months of bloodshed. The negotiators also agreed on the withdrawal of all heavy weaponry, the release of all prisoners and the delivery of humanitarian aid to devastated cities in eastern Ukraine. Western leaders voiced skepticism over Russia's commitment to the deal. A previous 10-day cease-fire, which each side repeatedly accused the other of violating, yielded few results at the negotiating table.

Ukrainian President Petro Poroshenko's office on Saturday said he and Russian President Vladimir Putin had discussed steps "for giving the cease-fire a stable character" in a telephone conversation.

But, it said, both leaders assessed the cease-fire as having been "fulfilled as a whole."

Fighting between pro-Russian rebels and Ukrainian government troops has ravaged the already teetering Ukrainian economy, claimed at least 2,600 civilian lives and left hundreds of thousands homeless, according to United Nations estimates.