By Erika Solomon
SANAA (Reuters) - Bursts of shelling threatened a fragile new truce in Yemen's capital Sanaa late on Tuesday as politicians scrambled to end the bloodiest fighting in eight months of anti-government protests.
Both government forces and troops loyal to General Ali Mohsen, who defected to pro-democracy protesters in March, vowed to stand by a ceasefire ordered by Vice President Abd-Rabbu Mansour Hadi.
But witnesses said two mortars hit the end of a street on Tuesday evening where thousands of protesters were camping out to demand an end to President Ali Abdullah Saleh's 33-year rule.
"The whole place shook with the explosion and clouds of dust shot up in the air when the second mortar hit," protester Badr Ali said.
The death toll has risen to around 70 since Sunday, when protesters' frustration boiled over at Saleh's refusal to accept a mediated handover plan. Saleh has been in Saudi Arabia since June, where he had surgery on injuries that he suffered in an assassination attempt.
The fighting between state troops and defected soldiers began after tens of thousands of protesters marched on Sunday close to a part of Sanaa controlled by government forces.
World powers fear that chaos in Yemen, home to al Qaeda's most powerful regional branch and adjoining the world's biggest oil exporter Saudi Arabia, could imperil oil shipping lanes and raise the risk of militant strikes on Western targets.
Despite the violence, opposition and government sources said talks were continuing over a Gulf-backed transition plan to ease Saleh out of office, from which Saleh has backed out three times.
U.N. mediator Jamal bin Omar and Gulf Cooperation Council Secretary General Abdbullatif al-Zayani arrived in Sanaa on Monday to boost efforts to get the deal signed.
A Western diplomat told Reuters mediators were trying to hang on to the positive direction the talks had been taking only a few days before the clashes.
During two days of chaos, Reuters reporters saw government forces using heavy fire against street marches and snipers shooting at protesters from rooftops.
Government officials and opposition groups have traded blame. But there appeared to be broad agreement that government forces had clashed with those of the defected General Ali Mohsen, who has pledged to defend protesters, after his men took control of territory previously under government control.
The opposition said Mohsen's troops took the area to head off security forces they believed would enter the protest camp.
A source at Mohsen's office said on Tuesday his forces would hold fire at the vice president's request, but that the protesters might be harder to control. "I don't think the youth protesters can be reined in until this regime leaves power."
Some 400 protesters have been killed since protests began in January.
The International Committee of the Red Cross (ICRC) said on Tuesday it had reports of shooting at al-Gomhori Hospital, one of Sanaa's main hospitals, as violence reached "unprecedented" levels in Yemen's capital.
Four defector soldiers were killed in street fighting with pro-Saleh forces on Tuesday, and two civilians died when three rockets crashed into a protest camp just after Tuesday morning prayers at around 5 a.m. (9 p.m. EDT on Monday), witnesses said.
"We were walking back from prayers. All of a sudden a mortar hit close by from out of nowhere, and some people fell down. And then a second one came and that's when we saw the two martyred," Manea al-Matari, a protest organizer, told Reuters by telephone.
The wounded were carried on blood-streaked stretchers to the field hospital in the camp, which the protesters have named "Change Square." Doctors said two had died, and two more protesters were killed by what they believed were sniper shots.
Mohsen, a top Yemeni general, dealt a major blow to Saleh when he and his troops defected after an attack on demonstrators by security forces in March that killed 52 people.
"There are spoilers on both sides who are not looking for a compromise or maybe aren't getting what they want from a compromise," said April Longley Alley, senior Arabian Peninsula analyst at the International Crisis Group in Abu Dhabi. "Maybe they feel they could achieve more by escalating right now."
Separately, five militants and a soldier were killed in fighting on Tuesday near Zinjibar, capital of Abyan province, where al Qaeda-linked militants began seizing territory in March, a local official and residents said.
(Additional reporting by Stephanie Nebehay in Geneva; Editing by Kevin Liffey)