Four explosions have been heard in Yemen's capital, Sanaa, as fighting worsens across the country and security forces continue their clampdown on anti-government protesters.

Information about the blasts, reported late on Monday, was sketchy, but Jamila Rajaa, an activist in Yemen, told Al Jazeera "it seems that there have not been any casualties".

Sanaa also was the scene of fresh clashes between Yemeni security forces and supporters of Sadiq al-Ahmar, the leader of the powerful Hashad tribe, sources told Al Jazeera.

The fighting, near al-Ahmar's residence, came a day after a ceasefire was declared between him and Ali Abdullah Saleh, the country's embattled president.

Dozens have been killed across the country in the last 24 hours.

At least 30 people were reportedly killed by military-jet fire in the southern city of 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.

'City devastated'

According to residents, the jet response targeted positions held in Zinjibar 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.

Opposition leaders accuse Saleh - who is under pressure to quit and end his 33-year rule - of allowing Zinjibar to fall to al-Qaeda and allied fighters in order to raise alarm in the region that would in turn win him support.

Elsewhere in Yemen, at least 20 people were killed in the southern city of Taiz after soldiers opened fire indiscriminately on a protest camp, a source said.

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 tents of the protesters and fired water cannons and tear gas at the crowd.

Security forces arrested dozens of people on to head off plans for another rally in Taiz, where Saleh's troops have parked armoured vehicles in "Freedom Square".

"All the responses that came from Taiz have been very strong and very determined to carry on with the revolution, activist Rajaa told Al Jazeera on Monday.

"They are determined that all of Taiz is going to become Liberty and Freedom Square. The mood is very tense, the mood is very determined that they are going to carry on with the planned demonstrations."

More clashes

The latest unrest came days after troops loyal to Saleh clashed with Hashed tribesmen who support the opposition.

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.

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.

"The protesters have been againts the initiatives and the agreements from the beginning," Rajaa told Al Jazeera. "They knew that Ali Abdullah Saleh will not step down and fight until his very last breath."

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.

Source: Al Jazeera and agencies