SANAA, Yemen - Yemeni forces opened fire on a protest camp and killed more than 20 demonstrators Monday in the southern city of Taiz while government warplanes launched airstrikes on another southern town seized by radical Islamists.

The new attempts to suppress the uprising against President Ali Abdullah Saleh with overwhelming force, following a weekend when high-level military defectors formed a united front in support of the protesters, all pointed to the longtime leader's increasingly tenuous grip on power.

Early Tuesday, residents said multiple explosions were heard in several parts of Sanaa, apparently from heavy weapons and shells. They said clashes were in progress in the capital. There were no immediate details of who was fighting or whether there were casualties.

More than three months of mass street protests have posed an unprecedented threat to Saleh's 33-year rule, splintering his security forces and battering the country's already frail economy. The U.S. has moved away from it former ally despite fears that his fall could leave room in this rugged corner of the Arabian Peninsula for an active al-Qaida franchise or other militant Islamist groups to take power.

Saleh has responded to protesters who say they seek democratic reforms with a mix of promised concessions and bloody crackdowns, such as Monday's attack in the city of Taiz that left at least 20 protesters dead. He has also long raised the specter of an Islamist takeover of Yemen to solicit international funds and rebuff calls that he stand down.

Yemen's weakly governed provinces are known as a haven for al-Qaida in the Arabian Peninsula, one of the group's most active branches, and other Islamic militant groups like the one that overran the southern town of Zinjibar last week.

Government jets bombed the town's outskirts Monday, the loud booms sending up columns of smoke, resident Ali Dahmis said by phone. He said the army was targeting residential areas.

The airstrikes were the government's hardest hit yet against the Islamists since hundreds of them streamed in Friday, seizing banks and government buildings. Military units battled them overnight and into Monday.

"The sound of explosions and bullets are rattling the city," resident Waleed Mokbal said. "The exchange of gunfire is nonstop."

The death toll since Saturday rose to 34, a medic at al-Razi hospital said, speaking on condition of anonymity because he was not permitted to talk to journalists. He said the dead included militants, civilians and soldiers. Militants shot dead four soldiers Monday after stopping them at a checkpoint outside the city, the medic said.

It remains unclear whether the Islamists who seized Zinjibar are connected to al-Qaida. Other armed Islamist groups have sought refuge in the area, some whom fought in Iraq and Afghanistan, others who fought with Saleh's government in a 1994 civil war with the south. Those militiamen demanded payback for their help and received positions in the security forces and civil service.

Residents said the men looted banks, making them look more like criminal gangs than ideological fighters.

"Not all armed Islamist groups in that area are al-Qaida," said Gregory Johnsen, a Yemen expert at Princeton University. "Whether they are working together in that area, it's such a murky situation that we just don't know."

The militants appeared to be the same group that seized the nearby town of Jaar in early April.

A group of high-level military officers who defected to the opposition accused Saleh Sunday of allowing the takeover, saying he sought to spread fears that Yemen without him would become "another Somalia."

The group of nine, which include high-ranking military commanders and former interior and defense ministers, issued a joint communique calling on all army units to help topple the president.

Johnsen, the Yemen expert, said it is unclear what portion of Yemen's armed forces the group commands. Elite units such as the Republican Guard, which Saleh's son Ahmed commands, have largely stood by the president. And to date, army units that have abandoned the president have kept their guns out of the effort to push Saleh from power.

Still, the statement is a slap, Johnsen said.

"If these guys are coming together to form some sort a unified command, particularly since these are many of the people Saleh has relied on in the past, that is very worrying for the president," he said.

Yemen's unrest has veered dramatically in the past week.

A U.S.-backed mediation effort by Yemen's powerful Gulf neighbors for Saleh to leave power in exchange for immunity from prosecution failed. Then five days of street battles in the capital between Saleh's security forces and fighters from the country's most powerful tribal confederation left 124 people dead before the sides reached a tenuous cease-fire.

In Monday's attack on the square in Taiz where protesters have camped out for weeks, security forces tried overnight to clear the area with water cannons, tear gas and stun grenades, sending thousands fleeing.

Forces from the Republican Guard then moved in before dawn with tanks, said Sadek al-Shugaa, head of the protest camp's field hospital. Soldiers and men in civilian clothes attacked the remaining protesters, setting fire to some tents and bulldozing others without checking whether anyone was still inside, two witnesses said.

One of the witnesses, Mohammed al-Zarafi, said government forces set tents alight with injured protesters inside.

The other witness, Boushra al-Maqtali, said the army took over the area.

"The artillery units are occupying the whole space to make it impossible for the youth to return to the square," she said.

Troops also attacked the Majeedi Hotel, which overlooks the square, detaining journalists and posting snipers on the roof to fire on protesters, al-Shugaa said.

Most of those injured had critical gunshot wounds to the head, chest and neck, he said. Security forces dragged away several dozen of the injured.

The U.S. Embassy in Sanaa condemned the attack as an "unprovoked and unjustified attack on peaceful demonstrators."

Late Monday, security forces shot dead another protester during a march through the city, Al-Shagaa said.