By Erika Solomon and Mohammed Ghobari
SANAA (Reuters) - Gunfire hit a protest camp in southern Yemen late on Thursday, after fighting in northern districts of the capital broke a truce aimed at ending the worst violence since a revolt against President Ali Abdullah Saleh began eight months ago.
South of the capital, in the protest hotbed of Taiz, activists said their protest camp, where demonstrators have camped out for months to demand Saleh's removal, was under attack.
"As dusk set in there was heavy gunfire on the camp, the bullets were flying over our heads. Now we're seeing armored vehicles approaching the square and we're afraid they are going to storm it," said activist Bushra al-Maqtari.
Northern districts of Sanaa were rocked by fighting early in the day, but the city later settled into a tense calm, with extra checkpoints set up by warring factions and many streets still deserted.
A Reuters reporter at the scene said shelling and gunfire had engulfed part of north Sanaa at dawn as troops and tribes loyal to the president battled armed followers of powerful tribal leader Sadeq al-Ahmar, who supports opposition demands for an end to Saleh's 33-year grip on power.
A doctor treating casualties said two opposition fighters had been killed and six wounded in the violence.
Many residents fled their homes as the fighting intensified, shattering three days of calm after Saleh ordered a ceasefire on his surprise return to Yemen on Friday.
"I only returned to the streets two days ago after clashes stopped, but I'll stick to the south of Sanaa today because it's safer," said ice-cream vendor Abdullah al-Wasabi. "We're tired of this crisis and we're losing our business, while these tribes and the president's soldiers don't tire of fighting."
The truce halted a week of fighting that killed more than 100 people and revived fears that Yemen, which borders top oil exporter Saudi Arabia, would tip into civil war.
The United States and Saudi Arabia fear lawlessness in Yemen could embolden the al Qaeda wing based there, endangering Western interests in the Gulf and oil transit routes through the Red Sea.
Even before protests paralyzed the impoverished state, Yemen was grappling with a tenacious wing of al Qaeda, a separatist insurgency in the south and a sectarian rebellion in the north.
In the war-torn northern governorate of Saada, which borders Saudi Arabia, aid group Medecins Sans Frontieres (MSF) said it had been forced to suspend its activities on Monday after local authorities imposed new conditions on humanitarian organizations operating there.
MSF said the conditions, which included a ban on international staff supervising operations, would "greatly affect" its ability to guarantee the quality and effectiveness of its work. "We had no choice but to suspend our activities," it said in a statement.
Until last week, Saleh had been recuperating in Riyadh from a June assassination attempt. Western diplomats had pressed him to stay in Saudi Arabia while they struggled to push through a long-stalled plan to transfer power.
The president has hung on in the face of nationwide protests that drew strength from popular revolts in Egypt and Tunisia.
Former Saleh allies, such as Ahmar and Ali Mohsen, a general who defected to the opposition in March, have deserted him. The conflict has divided Sanaa into warring zones of influence.
Thursday's fighting set Ahmar's tribesmen against state security forces and troops from the elite Republican Guard, commanded by Saleh's son.
During the lull politicians and diplomats had scrambled to revive a Gulf-brokered plan under which Saleh would stand down. Gulf nations and Western powers have been exasperated by Saleh's repeated last-minute refusals to sign agreed transition deals.
(Additional reporting by Khaled al-Mahdy; editing by Tim Pearce)