At least 18 people have been killed in fighting between troops loyal to Yemen's embattled leader and rival forces, including eight supporters of a powerful tribal chief who defected to the opposition in March.
The fighting, the worst in the capital Sanaa in weeks, has revived fears of civil war in the strategically located nation on the southern corner of the Arabian Peninsula.
Mortars, rockets and heavy machine-guns were used in the hours-long battle in the northern sector of the city close to the international airport.
A series of blasts shook the city for hours, forcing residents in many parts to take shelter in basements.
The fighting has deepened fears that Yemen, the Arab world's poorest nation, is headed for civil war, a grim prospect for the nation's conflict-fatigued 23 million people.
Firearms have traditionally been readily available in Yemen, where owning a rifle is a rite of passage for most young males.
Many Yemenis also have military experience from serving in the army and fighting in the nation's many domestic wars.
With central authority ranging from weak to non-existent outside Sanaa, it is not uncommon for tribesmen to have heavy machine-guns, anti-aircraft guns, mortars and rocket-propelled grenades.
President Ali Abdullah Saleh is accused by many Yemenis of pushing the country into civil war by tenaciously clinging to power in the face of eight months of massive protests across the country, the defection to the opposition of key tribal and military allies and mounting international pressure on him to step down.