SANA, Yemen — A cease-fire between government forces and opposition tribesmen in Sana, the capital, broke down on Tuesday, renewing fears that the country’s continuing political stalemate could drag it into civil war.

The fighting came a day after government forces pounded a major coastal city with airstrikes to dislodge Islamic militants and, to the west, disrupted a large antigovernment demonstration in the city of Taiz in clashes that killed at least 20 protesters.

Artillery explosions and machine-gun fire echoed across the center of Sana from late Monday into Tuesday morning as fierce fighting shattered a tenuous truce that had lasted fewer than two days.