(CNN) -- Government troops fought street battles with one of Yemen's leading tribes in the capital Thursday, leaving dozens dead as prosecutors sought the arrests of several tribal leaders.

A senior Defense Ministry official said that more than 28 people were killed in an explosion at a weapons depot in Sanaa during clashes with members of the al-Hashid tribe, which has turned against longtime President Ali Abdullah Saleh.

The official, who asked not to be named because he is not authorized to speak to the news media, said the tribe shelled the area around the depot with heavy artillery. But the tribe rejected the accusation, saying it was choosing targets cautiously and had not attacked civilians.

Government forces, meanwhile, hit an opposition-controlled television station with rocket-propelled grenades overnight, taking it off the air, witnesses in the Arab nation said Thursday. Government troops blocked the roads leading into the capital to prevent other tribal forces from joining the battle.

Witnesses said the fighting subsided considerably after nightfall, but gunfire still crackled across several parts of Sanaa. The fighting has raised fears of a full-blown civil war in Yemen, an impoverished, arid, mountainous nation that has been a U.S. ally in the battle against the al Qaeda terrorist network.

Before dawn Thursday, residents of Sanaa's al-Hasaba neighborhood called out from the minarets of local mosques for an end to the shelling for the sake of civilians. Those left in the neighborhood were without power, and some homes were reported to have been hit, witnesses reported.

Other witnesses said government forces bombarded the homes of Sadeq al-Ahmar, leader of the al-Hashid tribe, and his family from the mountains around Sanaa, where major military installations and arsenals are located. A makeshift hospital in Change Square, near Sanaa University, treated large numbers of casualties throughout the night, including tribal fighters and civilians.

The office of Yemeni Attorney General Abdullah Al-Olufi issued arrests warrants Thursday for al-Ahmar and eight other people, including several of his brothers and other tribal leaders. Abdulqawi Al-Qaisi, a top aide to al-Ahmar, said that none of the tribal figures has been arrested.

"The Yemeni government (is) not able to capture a chicken in the streets, let alone the most powerful tribal leader in Yemen," Al-Qaisi said. He said Saleh "will face the fate of previous criminal leaders," like ousted Egyptian strongman Hosni Mubarak.

"Sheikh Sadeq al-Ahmar is a hero in the eyes of Yemenis, and Saleh is the criminal," he said.

The fighting broke out after a regionally brokered deal calling for Saleh to leave office fell through on Sunday. Saleh himself is a member of the al-Hashid tribe, a huge and powerful entity with many strands.

Al-Qaisi said 51 people, including tribesmen and civilians, died in overnight clashes. Yemen's government-run television reported Thursday that four people were killed and 11 were injured by a shell fired by tribe members.

Some analysts expressed doubt Yemen will be able to avoid a civil war.

"We've seen the last (Middle East) dictator leave voluntarily," said Michael Rubin, a regional expert at the American Enterprise Institute, a conservative Washington think tank. "Libya is the new model for Yemen, Syria and any other hangers-on."

Western leaders have been pushing peaceful change in the Arab world, similar to what happened in Egypt and Tunisia. But Rubin said other dictators have seen the fate of Mubarak -- who is now facing trial and a possible death sentence -- and Libyan leader Moammar Gadhafi, who is facing an onslaught from NATO air forces.

They "see there is no possibility of a peaceful retirement," Rubin said. Many of them also "believe that their country is their personal fight."

On Sunday, Saleh backed out of a deal to step aside in the face of months of protests against his 33-year rule. Opposition leaders had already signed the document, brokered by the regional Gulf Cooperation Council, but Saleh insisted that they sign it again in his presence.

After Wednesday clashes between tribesmen and government forces near Sanaa International Airport, the United States ordered families of government employees to leave the country, along with "certain nonemergency personnel," citing "terrorist activities and civil unrest."

All arriving flights were diverted to Aden on Wednesday, and no flights were taking off from Sanaa, said a transportation official who asked not to be identified because he is not authorized to speak to the media.

The flight restrictions were imposed as anti-regime tribesmen battling Yemeni forces occupied the government's news agency compound and the tourism ministry in the capital Wednesday, the latest regime entities to be taken over during this week's street battles, witnesses said.

Al-Ahmar embraced the anti-Saleh movement after a March protest during which dozens of demonstrators were killed. Since then, more and more tribal members have turned their backs on the president as well.

The violence has attracted international attention. U.S. Secretary of State Hillary Clinton called Thursday for an immediate cessation of violence and urged Saleh to step down.

"We continue to support the departure of President Saleh, who has consistently agreed that he would be stepping down from power and then consistently reneged on those agreements, turning his back on the commitments that he made and disregarding the legitimate aspirations of the Yemeni people," Clinton said.

British Foreign Secretary William Hague condemned the latest fighting and urged Saleh to sign the deal that faltered over the weekend, under which the Yemeni leader would step aside after 30 days.

"The agreement brokered by the Gulf Cooperation Council offers the chance for peaceful progress in Yemen, and therefore we urgently reiterate the United Kingdom's call to President Saleh to sign the agreement, allowing for a peaceful transition and preventing further bloodshed," Hague said in a statement issued Thursday evening.

Britain reduced staffing at its embassy "to a level sufficient only to work on the most pressing and vital British national interests," and repeated its warning to its citizens to leave the country "while commercial flights are still operating."

Saleh has led Yemen since 1978 but has faced growing calls for his ouster amid the protests that have swept the rest of the Arab world since January's revolt in Tunisia. He has been a leading ally against al Qaeda, which has a Yemen-based branch that has claimed responsibility for two attempted attacks on the United States.

America's main concern regarding Yemen is that the country could become an al Qaeda stronghold if Saleh falls, Rubin said -- but U.S. policymakers also "have to recognize that al Qaeda rose under Saleh," he said. "Keeping him in doesn't keep al Qaeda out."

CNN's Mohammed Jamjoom and Alan Silverleib and journalist Hakim Almasmari contributed to this report.