At least 30 people have been killed by military jet fire in Yemen's southern city, Zinjibar, which is said to be controlled fighters linked to al-Qaeda.
The air attack on Monday appeared to be in response to Sunday's takeover of the city by 300 alleged al-Qaeda fighters and an overnight ambush that killed at least six Yemeni soldiers and injured dozens more who were travelling to the southern city.
"Civilians found a military car and an armoured vehicle. They were destroyed, and the bodies of six soldiers were found on the roadside," Ayman Mohamed Nasser, editor-in-chief of Attariq, Aden's main opposition paper, told the Reuters news agency by telephone.
According to residents, the warplane response targeted positions held by the fighters, but also hit buildings in the town of 20,000, killing at least 13 people earlier in the day and more than 17 later on.
"The city is devastated. All of its residents have left. Even the dogs, animals and donkeys have abandoned it," said an opposition member in the city who asked to be named as Ali.
Later on Monday night, four explosions were heard in Yemen's capital. More information about the blasts is not yet available.
Opposition leaders accuse Ali Abdullah Saleh, Yemen's embattled president under pressure to quit and end his 33-year rule, of allowing Zinjibar on the Gulf of Aden, to fall to al-Qaeda and allied fighters in order to raise alarm in the region that would in turn win him support.
'Run over by bulldozers'
Elsewhere in the troubled country swept by anti-government protests, at least 20 people were killed in the southern city of Taiz after soldiers opened fire indiscriminately on a protest camp, a source said.
They did not give further details on the violence in Taiz, but said the death toll was likely to rise.
Ashraf Khandari, a journalist based in Aden, said protesters were sprayed with live bullets and hot water, adding that "a lot of people" had been killed.
The latest unrest came days after troops loyal to Saleh clashed with Hashed tribesmen who support the opposition.
A tenuous truce was reported on Sunday, but unrest erupted when security forces tried to storm Taiz's "Freedom Square", where hundreds of anti-government demonstrators have been camped for days.
"Most of the wounded were hit by live bullets, but some were run over by bulldozers," a medical source from a field hospital told Reuters.
Al Jazeera correspondents said early on Monday that the security forces set fire to some tents of the protesters and fired water cannons and tear gas at the crowd.
Security forces arrested dozens of people on Monday to head off plans for another rally in Taiz, where Saleh's troops have parked armoured vehicles in "Freedom Square".
Separately, rocket attacks by government forces were reported in the tribal area of Arhab in south Yemen where fighting has taken place in the past, Al Jazeera's correspondents said.
People were fleeing the area, but there were no reports of casualties as yet.
Saleh has refused to sign a deal, mediated by Gulf Arab states, to start a transition of power aimed at averting civil war in Yemen.
A breakaway military group called for other army units to join them in the fight to bring down Saleh, piling pressure on him to end his rule over the destitute country.
Generals and government officials began to abandon Saleh after deadly crackdowns on protesters started in force in March.
There have been no major clashes yet between the breakaway military units and troops loyal to Saleh.
Yemen borders Saudi Arabia, the world's biggest oil exporter, and sits along a shipping lane through which about three million barrels of oil pass daily.