SANAA A breakaway military group called on Sunday for other army units to join them in the fight to bring down Yemen's President Ali Abdullah Saleh, piling pressure on him to end his three-decade rule over the destitute country.
Opposition leaders separately accused Saleh of allowing the city of Zinjibar, on the Gulf of Aden, to fall to al Qaeda and Islamists militants in order to raise alarm in the region that would in turn translate to support for the president.
Despite global and regional powers demanding he step down, Saleh has refused to sign a deal, mediated by Gulf states, to start a transition of power aimed at averting civil war that could shake the region that supplies the world with oil.
"We call on you not to follow orders to confront other army units or the people," the breakaway units said in a statement read by General Abdullah Ali Aleiwa, a former defence minister.
In Sanaa, a tenuous ceasefire appeared to be holding after nearly a week of fighting between Saleh's security forces and a powerful tribal group that left at least 115 dead and forced thousands to flee the capital for safety.
Residents in Zinjibar, about 270 kms (170 miles) southeast of the capital, said armed men likely from al Qaeda had control of the city in the flashpoint province of Abyan.
"About 300 Islamic militants and al Qaeda men came into Zinjibar and took over everything on Friday," a resident said.
Three militant gunmen and three civilians have been killed in fighting against locals, who have been joined by a few government soldiers, trying to take the city back from the al Qaeda group and Islamists, medical sources said.
Nearly 300 Yemenis have died over the past few months as the president has tried to stop pro-reform protests by force.
Generals and government officials began to abandon Saleh after a 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.
Opposition groups and diplomats have accused Saleh of using the al Qaeda threat to win aid and support from regional powers seeking his government's help in battling the militants.
Fears are growing that Yemen-based al Qaeda in the Arabian Peninsula (AQAP) will exploit such instability, analysts said. The United States and Saudi Arabia, both targets of attacks by AQAP, are worried that growing chaos is emboldening the group.
Yemen borders Saudi Arabia, the world's biggest oil exporter, and sits along a shipping lane through which about 3 million barrels of oil pass daily.
In Sanaa, pedestrians and cars returned to the streets where Saleh's security forces battled members of the powerful Hashed tribe led by Sadeq al-Ahmar in the bloodiest fighting since pro-democracy unrest erupted in January.
Ahmar's men handed back control of a government building to mediators as part of the ceasefire deal, witnesses said.
It was the first building seized by the tribesmen that was handed back under a truce brokered on Saturday intended to normalise life in the capital after street fighting with mortars, machineguns and rocket-propelled grenades.
Electronics stores, perfume sellers and other businesses were open but there were few customers, with many residents keeping tight hold on their cash in case fighting flared up again and they needed to quickly buy essentials.
"Business is very bad. We have had to sack some workers. There is no money," merchant Muthar Abdel-Rahman said.
The truce also extends to areas outside of Sanaa where tribesmen have clashed with the president's Republican Guards and air force fighters have strafed armed tribesman with bombs.
Some Guards members in southern Damar at the weekend joined the opposition, tribal sources said.
Despite the truce, analysts say fighting may start again, given the animosity between the various armed groups and growing popular anger at Saleh for not ending his nearly 33-year-long rule which has brought Yemen to the brink of financial ruin.
"We are still here to bring down this regime, even if it takes another week, another month or another year," Yusra al-Abssi said at a protest camp.
Yemen is the poorest country in the Arabian Peninsula, with about 40 percent of its residents living on less than $2 a day.
The crisis has cost the $31 billion GDP economy as much as $5 billion and immediate aid is needed to prevent a meltdown, Yemen's trade minister told Reuters on Saturday.
International negotiators have become exasperated with Saleh, saying he has imposed new conditions each time a Gulf-led transition agreement was due for signing, most recently demanding a public signing ceremony.
But global powers have little leverage on events in Yemen, where tribal allegiances are the most powerful element in a volatile social fabric and the fighting already appears to be playing out along tribal, quasi-feudal lines.
(Additional reporting by Mohammed Ghobari in Sanaa, Mohammed Mukhashaf in Aden; Writing by Jon Herskovitz and Firouz Sedarat in Dubai; Editing by Louise Ireland)