Twelve fighters were killed in northern Yemen in ongoing clashes between Shiite tribesmen allied with the country's former leader and ultra-conservative Sunnis, security officials said, as the new president grapples with challenges to his authority.
Tensions have long existed between Salafi Islamists, who are Sunni Muslims, and former Hawthi rebels, who are Shiite Muslims.
Nearly 200 people died in fighting late last year between the two sides in northern Saada province.
The latest clashes in neighbouring Amran province revealed a new alliance between Hawthis and loyalists of former President Ali Abdullah Saleh, whom the group fought in a costly six-year war until a cease-fire was reached in early 2010.
They also underline the larger regional rivalry between Iran and Saudi Arabia for influence in northern Yemen, an impoverished nation on the southern tip of the Arabian Peninsula, at the doorstep of several oil-producing Gulf nations.
Residents trapped in street fighting in the city of Raada in Amran say the Salafis from the opposition Islah Party were fighting Hawthis teamed up with Saleh loyalists, the two sides firing at each other with rocket-propelled grenades and automatic weapons.
In another sign of a new alliance, Sheikh Ali Sinan al-Ghouli, a Saleh supporter, said he travelled to the area to assist the Hawthi rebels.
The clashes come a week after Yemen's government installed new governors from the Islah Party in Amran, about 45 miles north west of the capital Sanaa, and two other provinces around Saada where Hawthis are concentrated.
According to military analyst Mohammed Ahmed al-Subayhi, the decision by President Abed Rabbo Mansour Hadi to appoint Islah Party members as governors reflects a larger regional rivalry between Saudi Arabia and Iran.
"Saudi Arabia believes that any Hawthi expansion in the region enhances Iran's role in Yemen and poses a threat to the kingdom," Mr al-Subayhi said.
The retired military general added that the Sunni Hashid tribe in Amran is closely aligned with Saudi Arabia.
Shiite-led Iran and Yemen's powerful Sunni Muslim neighbour to the north, Saudi Arabia, are bitter regional rivals and have competing strategic interests in the Gulf.
Saudi Arabia was a key player in brokering the deal that forced Mr Saleh to hand over power to Mr Hadi in February, after nearly a year of protests against him.
In July, Mr Hadi warned Iran to stop spying in his country in the wake of claims that an Iranian espionage operation had been uncovered. The US ambassador in Yemen has also repeatedly said that Iran is meddling in the country and playing on specific factions, a reference to Hawthi rebels in the north.
Also, security officials said a suspected al Qaida suicide bomber wounded three people in the southern province of Aden, among them the commander of local tribes who fought alongside the government against the group earlier this year. Abdel-Latif el-Sayid was in critical condition, officials said.