An injured tribesman loyal to the Ahmar family was aided after clashes with the police in Sana, the capital of Yemen, on Tuesday.  WASHINGTON — Hopes for a peaceful settlement of Yemen’s political crisis receded further on Tuesday as street battles intensified between government security forces and opposition tribesmen, leaving at least two dozen people dead and turning part of the Yemeni capital, Sana, into a war zone.  The fighting, which started Monday and included shelling of the Interior Ministry, threatens to spread into a broader conflict and displace the peaceful protests that began in Yemen shortly after the uprisings in Tunisia and Egypt. The Obama administration is struggling to avert that and is now weighing using the United Nations to pressure Yemen’s embattled president, Ali Abdullah Saleh, to step aside.  Mr. Saleh refused Sunday, for the third time, to sign a deal to transfer power, despite enormous pressure from the United States and regional Arab leaders.  If Mr. Saleh still refuses to sign, the United States could press for United Nations sanctions aimed at Mr. Saleh and his family members, who occupy key posts in Yemen’s military and intelligence services, according to a senior administration official who spoke on the condition of anonymity. On Sunday, the White House counterterrorism chief, John O. Brennan, warned the Yemeni president in a phone call that “if he doesn’t sign, we’re going to have to consider possible other steps,” the official said.  The United States and Yemen’s Arab neighbors are deeply concerned that the worsening political stalemate is allowing Al Qaeda’s Yemeni branch — one of the terrorist group’s most dangerous and active affiliates — to operate more freely. Already, several provinces are outside the government’s control, and the economy is in free fall, with opposition-aligned tribes in eastern Yemen disrupting the flow of oil and electricity to the capital.  The fighting on Tuesday was especially ominous because several prominent tribal sheiks came under fire as they arrived to help mediate the conflict, and at least two were reported killed. Yemen’s powerful tribes are well-armed and their fighters are experienced. Tribal leaders from across Yemen had begun threatening to descend on the capital with thousands of warriors to strike back at Mr. Saleh, whom they blamed for the violence.  Many Yemenis fear the bloodshed could spiral into a broader war between Mr. Saleh’s supporters and tribesmen allied with the Ahmar family, whose house — located near several important government buildings in the Hasaba district — was at the center of the fighting.  The Ahmars are the leaders of Yemen’s most powerful tribal confederation, known as Hashid. Hamid al-Ahmar, a telecom mogul, has long been a rival to the Yemeni president and is the most visible face of the political opposition that would inherit power if Mr. Saleh signs the agreement.  Another danger is an intertribal conflict. “This is what Saleh would like to start — a war between the tribes,” said Muhammad Abdel Qadhi, a sheik in Mr. Saleh’s own tribe, the Sanhan.  The protesters, who mostly remained camped out on the street in the area of the capital they have renamed “Change Square” on Tuesday, say Mr. Saleh is deliberately fomenting war so that their uprising will no longer be seen as a peaceful one, and his own continued rule will seem necessary so as to maintain calm.  The sound of exploding mortar shells could be heard throughout the capital on Tuesday, witnesses said. Opposition tribesmen shelled the Interior Ministry and other buildings for hours, and set up barricades in streets nearby, witnesses and local news reports. Much of the capital was deserted as residents cowered inside.  Each side blamed the other for the outbreak of fighting. There were varying death tolls, some as high as 40, with more than 150 said to be wounded. Yemen’s Interior Ministry released a statement saying that 14 soldiers were killed Tuesday, along with a family of five civilians whose house was struck by a rocket-propelled grenade. A prominent doctor, Abdul Wahab Al Anesi, said he had learned of 24 deaths of civilians and tribesmen from several hospitals in the city, but that could not be confirmed independently.  A group of tribal sheiks who arrived at the Ahmar family compound to mediate a cease-fire issued a statement to local reporters saying they had come under fire almost immediately after arriving, despite repeated efforts to coordinate by phone with the president’s loyalists. “We decided to stop the mediation and hold the president fully responsible,” the statement said.  So far, the high-ranking generals who defected to the opposition in March — with their armed forces — have not become involved. If that changes, the conflict could quickly become far bloodier, possibly spiraling into a full-scale civil war.  A political settlement has been elusive, in part, because Mr. Saleh’s loyalists and the opposition deeply distrust each other. American and Arab diplomats have focused on urging Mr. Saleh to sign the agreement, under which he would transfer power in exchange for immunity from prosecution for himself and his family members.  But even if he does so, the two sides must then work out the details of a transition, a process that could be prolonged and fraught with tension. The 30-day timetable for Mr. Saleh to leave office does not start until they complete the agreement.  Laura Kasinof and Helene Cooper contributed reporting.
The efforts to negotiate a settlement of the political crisis appeared more remote than ever after the clashes in Sana.