More than 400 tribal gunmen took over Taiz in southwest Yemen, eyewitnesses there said.
The gunmen had been clashing with Yemeni security forces near the city's Republican Palace and eyewitnesses said they are now in control of the city. The palace is not far from the city's Freedom Square -- a focal point of anti-government protests.
Government forces have been regrouping in an effort to re-enter the city. Yemen's government has faced international criticism for excessive use of force against anti-regime protesters and the deaths of anti-government demonstrators in Taiz.
Limited options in Yemen
What's next for Yemen?
Transfer of power in Yemen
The fighting intensified as President Ali Abdullah Saleh recovers in Saudi Arabia from burns over 40% of his body and a collapsed lung, a U.S. government officials briefed on the matter said.
The fate of the embattled leader -- and whether he will return to the conflict in Yemen -- remains uncertain.
Saleh was injured Friday in an attack at his presidential compound. An Arab diplomatic source with knowledge of Saleh's condition says one shrapnel wound is 7 centimeters (2.75 inches) deep.
Fighting between government and tribal forces has raged for weeks in the Yemeni capital of Sanaa, where thousands of anti-government protesters have been pressuring Saleh to give up power since January. And there has been unrest elsewhere.
At least 15 people were killed in Abyan Monday night and Tuesday morning in clashes between Islamic militants and security forces, according to a security source in Abyan.
Al Qaeda in the Arabian Peninsula has a significant presence in the southern province of Abyan, a U.S. official said. It's also home to an Islamic militant movement that has targeted government troops in recent days.
The number includes nine soldiers, four militants and two civilians in Zinjibar and other suburbs in the province, according to the source, who asked to not be named because he is not authorized to speak to the media.
There was violence along the Saudi-Yemeni border, where there have long been concerns of infiltration by militants.
A man attempting to cross the border from Saudi Arabia into Yemen early Tuesday shot dead two security officers and injured another, the state-run Saudi Press Agency said on Tuesday.
The man, described as an "infiltrator" who tried to cross the metal and sand barriers at the border, fired at the security officers when they approached him. Security personnel tracked the man and he was killed in an exchange of fire.
The turmoil in Yemen reached a pinnacle Friday, when a mosque in Saleh's presidential compound was attacked. Yemen's state-run news agency SABA reported last week that three guards and an imam were killed, citing a source in Saleh's office.
According to Western diplomats, the attack came from a bomb. Yemeni investigations are "focusing on what happened inside the mosque," not a rocket or mortar attack, diplomats said Monday. One diplomat said the bombing was not a suicide bombing and that the Yemeni investigation "is still ongoing."
But last week, a Yemeni official who asked not to be named told CNN that Saleh was in the mosque when two "projectiles" were fired during Friday prayers.
Supporters of Sadeq Al-Ahmar, leader of the Hashed tribe and an opponent of the Yemeni government, were suspected in the attack.
Yemeni security forces shelled Al-Ahmar's home Friday in response to the attack, leaving 10 people dead and 35 others wounded, according to Fawzi Al-Jaradi, an official with the Hashed tribal confederation.
After Saleh went to Saudi Arabia for treatment, the tribal leader and Vice President Abed Rabbo Mansour Hadi -- Yemen's interim leader -- agreed on a cease-fire, said Abdulqawi Al-Qaisi, spokesman for the Hashed leader.
Yemen's largest opposition bloc has vowed to prevent Saleh from returning.
"The Yemeni people will do all in their power to not allow Saleh to re-enter the country," Joint Meeting Parties spokesman Mohammed Qahtan said Sunday.
A U.S. government official said Monday he can't imagine the Saudis letting Saleh go back. He said it is critical that the Saudis press Saleh to accept a Gulf Cooperation Council deal offering him immunity in exchange for stepping down.
Saudi state-run Ekhbariya television reported Monday that Saleh had undergone two operations in Saudi Arabia and it reported he would return to Yemen after he recovers.
Christopher Boucek, a Middle East expert at the Carnegie Endowment for International Peace, said he doubts Saleh will go back.
"The regime still maintains that he will return, and they say he's going to return within days, if not weeks," Boucek said. "But there's really no option I see for how he can go back and still be president."
A U.S. official told CNN Monday that the unrest makes U.S. counterterrorism efforts in Yemen "more difficult."
"We rely on the Yemeni government as partners," the official said. "The more the government is distracted by the political unrest, the more difficult it is for us."
The Yemeni government has had a "big impact on acquiring information on AQAP," the official said, referring to al Qaeda in the Arabian Peninsula. "If that information flow slows or stops, it inhibits our ability to gather information."
Two opposition leaders in Yemen expressed cautious support Monday for Hadi, the vice president, while Saleh is away.
"We do not have any problem if Hadi takes control of the government. He is respected by the people," said Tawakkul Karman, adding that Hadi "must use this historic moment to enter Yemen's history as a leader and revolutionary."
But she warned that if he does not "conduct immediate reforms, the youth protesters will go against him the same way they did against Saleh. It's Hadi's choice to decide which door of history he wants to go through."
Ahmed Bahri, a senior official of the opposition Joint Meeting Parties, said that if Hadi can lead peaceful change, "we welcome it. If not, he should step aside and not stall the revolution."