Yemeni government troops in Sana'a are again fighting members of the Hashid tribe, just one of several forces, including militants and anti-government protesters, seeking to oust President Ali Abdullah Saleh.
The street battles followed a brief truce between the government and forces loyal to Sheikh Sadek al-Ahmar, and threatened once again to push Yemen toward civil war.
The two sides had tentatively achieved a cease-fire Sunday, but al-Ahmar loyalists accused the government of breaking the deal with renewed attacks on the sheikh's Sana'a compound. The government countered that fault lay with Hashid fighters who retook key government buildings in Sana'a.
Meanwhile, officials and medics say suspected militants have killed at least five soldiers near the southern town of Zinjibar, which was seized by fighters described as Islamist militants in recent days. Government forces have carried out air strikes over the town.
The underlying opposition demand - that President Saleh step down as outlined in a deal brokered by the Gulf Cooperation Council - remains unchanged. Saleh has three times rejected the plan.
Tom Finn, a journalist in Sana'a, described the fighting in the past days as the heaviest since protests began against Saleh four months ago.
"There was continuous machine gun fire and very loud, heavy explosions which were rattling the windows of houses across Sana'a," recalled Finn.
He says both sides are firing shells and anti-aircraft missiles at the other, ravaging part of the capital.
"The east of the capital has pretty much been evacuated. It's an area called Hasaba. It's a heavily, densely populated residential area which is now a ghost town," Finn said. "The houses are empty, the streets are filled with smashed up walls and concrete and bullet holes. It's essentially a battle zone in this neighborhood."
More residents were reported trying to leave the capital, after others fled during fighting between the two sides last week.
"What I'm seeing and hearing on the streets of Sana'a is sort of widespread panic and fear of what's coming in Yemen," said Finn.
The clashes in Sana'a and Zinjibar are but two of the fronts in the effort to force President Saleh from power. Witnesses in the southwestern city Taiz say more anti-government protesters were killed by troops Tuesday.
The government's deadly push to clear the city's central square began Sunday. The United Nations refugee agency says it has reports that more than 50 people have been killed in the crackdown in Taiz.
While the pressure on Saleh is clearly growing, whether his disparate opponents can, or even want to unite, is less apparent.
Tom Finn says the matter is dividing political protesters in Sana'a, who, inspired by uprisings in Tunisia and Egypt, have been seeking a democratic alternative to more than three decades of Saleh's authoritarian rule.
"There's mixed opinion of whether they want the support of these tribes, whether or not they want to see armed groups with them because generally speaking, one of the main mottos of their urban movement is that it is peaceful," Finn said.
He says a big worry for the political protesters now is that their movement is going to be "completely eclipsed" by the violence.
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