Strong explosions have been heard on the outskirts of the main rebel-held city in eastern Ukraine near the airport, raising new fears that a cease-fire signed two days ago is on the verge of collapse.

The cease-fire had appeared to be holding for much of the day on Saturday, but shelling took place on the edge of the port city of Mariupol late at night. The city council said one serviceman was wounded when a checkpoint was hit.

On Sunday morning, blasts powerful enough to be heard in downtown Donetsk came from the area near 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.