By Samia Nakhoul and Khaled Abdullah

SANAA/TAIZ (Reuters) – Yemeni forces loyal to President Ali Abdullah Saleh opened fire on protesters in the southern city of Taiz on Sunday, killing at least six people and wounding 120, hospital sources said.
A Reuters photographer at the scene said police fired live ammunition, tear gas and used water cannons to disperse demonstrators protesting outside a municipal building to demand the release of a fellow protester who was arrested on Saturday.

The clashes took place near Freedom Square where thousands of anti-government protesters have been camping since January to demand Saleh's overthrow. Police set two tents on fire in the square. Protesters hurled Molotov bombs and rocks at police.
In the capital Sanaa, seven explosions were heard on Sunday night in the district of Hasaba, the scene of week-long fighting between Saleh's forces and a rival tribe in which 115 people were killed, residents said.
There were no immediate details on the explosions, which appeared to have partially breached a truce between Saleh's forces and the powerful Hashed tribe led by Sadeq al-Ahmar in the bloodiest fighting since unrest erupted in January.

Ahmar condemned what he described as "Saleh's new massacre" against civilians in Taiz. Earlier on Sunday, his men handed back control of a government building to mediators as part of a ceasefire deal.
PILING PRESSURE
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 three-decade rule over the destitute country.

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 defense minister.

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.
Residents in Zinjibar, about 270 km (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 been killed 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 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.

(Additional reporting by Mohammed Ghobari in Sanaa, Mohammed Mukhashaf in Aden, Khaled Abdullah in Taiz; Editing by Maria Golovnina)