Yemeni officials say 18 people have been killed and at least 30 wounded in clashes between regime forces and fighters allied with activists demanding an end to Ali Abdullah Saleh's 33 years in office, officals and witnesses said on Monday
The officials said eight followers of a tribal leader who defected to the opposition in March were killed early Monday in clashes with forces loyal to Saleh in the Yemeni capital, Sanaa.
They said four civilians were killed in the fighting along with two government troops.
Three people were also killed when mortars exploded in a protest encampment in central Sanaa.
Also, a man was killed in the city of Taiz, when pro-government gunmen fired on protesters.
Witnesses said six civilians were killed in the fighting between government troops on one side, and fighters loyal to tribal leader Sheikh Sadeq al-Ahmar on the other. Ahmar's fighters are backed by a breakaway army unit led by a general from the same tribe, Ali Mohsen al-Ahmar.
A shell killed three people when it landed near a field hospital set up at "Change Square," where thousands of protesters have camped for months demanding Saleh step down, witnesses said.
At least 30 were wounded across Sanaa and in Taiz. The officials spoke on condition of anonymity because they were not authorized to speak to reporters.
Violence in Yemen has surged since Saturday as U.N. Security Council members consider a resolution expected to urge Saleh to step down under a peace plan hammered out by neighboring Gulf states.
Saleh has remained in office despite ten months of mass protests against his rule inspired by demonstrations across the Arab world. Opposition to him has turned increasingly violent and organized, threatening to plunge the country into all-out civil war.
Yemen, the Arab world's poorest country, also faces tribal conflict, violence from a strong regional al-Qaida wing, separatism in the south and sectarian conflict in the north.
Saleh, who says he is ready to step down but wants to ensure that control of the country is put in safe hands, has said he is relying on support from Russia and China to stop moves to force him to step down.
Speaking at a meeting of his security and military chiefs in Sanaa, he said Western countries with permanent seats on the Security Council had based their decisions on information gathered solely from the opposition.
The United States and Saudi Arabia, which shares a long and porous border with Yemen, fear that al-Qaida is taking advantage of the political vacuum to expand its influence.
Islamist militants linked to al-Qaida captured large swathes of southern Abyan province, including regional capital Zinjibar, earlier this year. The Yemeni army last month drove the militants out of Zinjibar, east of a strategic shipping strait through which some 3 million barrels of oil pass daily.
Tribal sources said at least five people were killed late on Sunday when tribesmen ambushed members of al-Qaida in Zinjibar. Local officials said security forces captured three suspected militants, including a Saudi national, in the incident.
"The tribal fighters ambushed the militants as they were transporting military supplies late on Sunday. The two sides fought, leading to the deaths of four militants and one tribesman," a tribal source told Reuters.
The tribesmen also destroyed a D-130 tank which militants had seized from the Yemeni army when captured Zinjibar earlier this year, the source said.
Suspected militants blew up Yemen's gas pipeline last week after an air strike killed a top al-Qaida leader and a number of other militants in southern Yemen.