Deadly fighting has spread across Yemen's capital as tribesmen joined battles between rival military units, raising fears among residents the country is descending into civil war.
At least fifteen people were killed in the battles that rocked the north of Sanaa on Thursday, the defence ministry's website and tribal sources said.
The clashes, now in their fifth consecutive day, reached the doorsteps of the US and British embassies in Sanaa, witnesses said.
The escalating tensions between troops loyal to Ali Abdullah Saleh, the Yemeni president, and opponents of his regime collapsed efforts on Wednesday by international mediators to promote a Gulf-initiated peace deal aimed at halting the political impasse that has gripped Yemen for months.
At least four civilians were killed when they were caught in the crossfire of the fighting that broke out early Thursday between Republican Guard troops commanded by Saleh's son Ahmed, and dissidents loyal to General Ali Mohsen al-Ahmar, witnesses and medics said.
They said two women and a man were shot by snipers positioned on rooftops near and overlooking Change Square, the base of the anti-government protesters.
Another man died from wounds sustained when a mortar shell smashed into the square. Nine people were also wounded in the blast and several tents set up by protesters caught fire, according to witnesses.
Medics said hundreds had been wounded, adding that most of the casualties were civilians caught in the crossfire or gunned down by Saleh loyalists.
"I can no longer open my store for fear of stray bullets, whether from the opposition or government forces. Every day there are many casualties due to the stray bullets," said 25-year-old Mohammed al-Jabiri, who owns a mobile phone shop in Sanaa.
Fighting which had been concentrated since Sunday in the city centre and at Change Square spread on Thursday to Sanaa's Al-Hasaba district, where gunmen loyal to powerful dissident tribal chief Sheikh Sadiq al-Ahmar traded fire with followers of Saghir bin Aziz, a tribesman loyal to Saleh, witnesses said.
Bin Aziz, who is from Yemen's most influential tribe Bakil, is also a Republican Guard officer and a member of parliament.
Witnesses said men loyal to Hemyar al-Ahmar, Sheikh Sadiq's brother, joined the battles and that shells were being fired from the building of Yemen's interior ministry towards his house and that of another brother Hussein al-Ahmar.
United Nations envoy Jamal Benomar told the AFP news agency late on Wednesday that the deteriorating security situation, and the reluctance of both sides to reach a political resolution, raises "the risk of civil war breaking out".
Gulf Cooperation Council chief Abdullatif al-Zayani is expected in New York on Friday to discuss the Yemeni crisis with GCC foreign ministers and international diplomats who are gathered at the UN for the annual General Assembly meeting, a Yemeni diplomat said, requesting anonymity.
The latest violence is the worst incident of bloodshed since a similar massacre killed 52 people in mid-March.
Saleh, who has been in power for 33 years, has since January faced protests over nepotism and corruption from reform activists inspired by the Arab Spring.
He left the country three months ago for Saudi Arabia where he has been recovering from a June 3 attack on his presidential compound.