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 May 27, 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 between the Nehm tribesmen and soldiers of the Yemeni Republican Guard, which started earlier on May 27.

The rising violence and unrest prompted a video statement, issued online on May 27, by the Assistant US Secretary of State Janice Jacobs urging American citizens "to depart Yemen while commercial transportation is still available."

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 US ally in the battle against the al Qaeda terrorist network.

The Office of the UN High Commissioner for Human Rights said the escalation in violence was alarming. Spokesman Rupert Colville said that the agency has received reports of dozens of civilian casualties, including women and children, in the fighting over the past few days.