(CNN) -- Five loud explosions rung out early Saturday in the Yemeni capital, residents said, amid a surge of violence pitting tribal forces against those loyal to embattled President Ali Abdullah Saleh.

The latest blasts appeared to be in the Al-Hasabah neighborhood of Sanaa, which is home to Sheikh Sadeq al-Ahmar, the leader of the powerful al-Hashid tribe whose forces oppose the government.

On Friday, a senior Yemeni defense official said that at least seven air force combat jets had bombed tribal forces opposed to the president.

The bombers were deployed east of Sanaa to the district of Nehm, where two military compounds had been overtaken earlier by tribal fighters, said the official, who was not identified because he is not authorized to speak to the media.

Ahmed Soufi, a senior adviser to Saleh, said 18 people were killed in the fighting.

Witnesses in Nehm said the airstrikes were carried out in several areas of the province northeast of Sanaa, including Al-Fartha Thoma and Beni Shokan.

The fighting started earlier Friday between the Nehm tribesmen and soldiers of the Yemeni Republican Guard.

The tribesmen said the soldiers attacked a village and tribal fighters, battling back, managed to take over military compounds. They said several Yemeni soldiers were killed but CNN could not verify casualties.

"The guards attacked one of our villages for no reason," said Sheikh Moqbel Najeeb, a tribal leader in the area. "We will not accept that and will fight back against anyone who tries to attack us."

The clashes in Nehm have been going on for the past month, Soufi said. He said the tribal fighters were encouraged by the actions of al-Ahmar.

Soufi said the fighting this week erupted after tribesmen were barred from entering Sanaa.

The Nehm district has about 35 tribes but they are not affiliated with al-Hashid.

Abdul Makik Ali, a Nehm tribal fighter said at least seven tribes joined to fight the Yemeni soldiers Friday.

"Our blood is not cheap and we will avenge from the government for every drop of Nehm blood that is shed," Ali said.

The rising violence and unrest prompted a video statement, issued online Friday, by the Assistant U.S. Secretary of State Janice Jacobs urging American citizens "to depart Yemen while commercial transportation is available." The message also outlined ways for people to contact the U.S. government, whether they planned to stay in or leave the Arab nation.

"Rest assured, the State Department is monitoring events in Yemen very closely," said Jacobs.

Saleh, who has ruled Yemen since 1978, has been clinging to power amid a wave of protests calling for his ouster in the past few months.

The simultaneous tribal battle began after a regionally brokered deal calling for Saleh to leave office fell through. Saleh himself is a member of the al-Hashid tribe, a huge entity with many strands.

After an incident in March during which dozens of anti-government demonstrators were killed, al-Ahmar embraced the anti-government demonstrators and broke ranks with the president. Since then, more and more tribal members have turned their backs on the president.

The recent fighting has raised fears of a full-blown civil war in Yemen, an impoverished, arid and mountainous nation that has been a key U.S. ally in the battle against the al Qaeda terrorist network.

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.

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 Saleh.

Government forces, meanwhile, hit an opposition-controlled television station with rocket-propelled grenades overnight, taking it off the air, witnesses 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 Office of the U.N. High Commissioner for Human Rights said the escalation in violence was alarming. Spokesman Rupert Colville said Friday that the agency has received reports of dozens of civilian casualties, including women and children, in the fighting over the past few days .

"We are deeply concerned that such violence may be pushing the country to the brink of a civil war," Colville said at a briefing in Geneva.

"We call on the government to stop the excessive and disproportionate use of force, to stop targeting activists, human rights defenders and journalists, and to seriously investigate all allegations of crimes committed by security forces."