By Mohammed Ghobari
SANAA (Reuters) - Yemeni soldiers opened fire on Shi'ite Muslim protesters trying to storm the cabinet building in the capital Sanaa on Tuesday, killing at least four, a Reuters photographer and medical sources said.
The anti-government activists loyal to the Houthi religious insurgent group said dozens were wounded in the bloodiest day of tense demonstrations that have gripped Sanaa for weeks and now threaten to descend into outright conflict.
A security source speaking on condition of anonymity said marchers "tried by force to storm the cabinet, so the forces guarding it undertook their legal duty to stop them."
Protesters have blocked the main road to Sanaa's airport and have held sit-ins for weeks at ministries in an attempt to oust the government and restore fuel subsidies.
The Houthis accuse the government of corruption, while the group's critics say it is trying to grab power and carve out a semi-independent state for itself in the north - something it denies.
While activists hold rallies in the capital, armed Houthi rebels in Northern al-Jawf province have fought government-allied tribes backed up air strikes over the last three days.
Both sides vowed defiance on Monday, as the International Crisis Group think tank said "Yemen's troubled transition is at a crossroads more dangerous than any since 2011," when mass protests ousted veteran autocrat Ali Abdullah Saleh and security around the country sharply deteriorated.
"The Houthi group may not and cannot continue its escalation and its disturbance of the public tranquillity and the undermining of security and stability in the capital," Salah's successor, President Abed Rabbo Mansour Hadi, said.
Houthi leader Abdel Malik al-Houthi said: "We vow to continue in our position and dismiss any questioning of our intentions. The solution is to respond definitively to the popular demands, and we are serious in these demands."
Separately, al Qaeda militants detonated a car bomb at an army checkpoint in the eastern Hadramawt province, setting off a firefight in which three soldiers and 10 militants were killed, a local security source said.
The United States and Yemen's energy-rich Gulf neighbours fear for the impoverished country's stability as it battles al Qaeda in the Arabian Peninsula, one of the fiercest affiliates of the global militant group, and struggles with sectarian rifts which have led to warfare in other Arab countries.
(Additional reporting by Mohammed Mukhashaf in Aden, Writing by Noah Browning; Editing by William Maclean and Andrew Heavens)