* Fighting erupts when militants attack military camp
* Warplanes bomb checkpoint held by al Qaeda militants (Adds more security developments)
ADEN- At least 25 people were killed on Monday when fighters from an al Qaeda-linked group attacked a military camp near the southern Yemen city of Lawdar, residents and local officials said.
The fighting erupted when fighters from Ansar al-Sharia launched a dawn attack on the camp, which is in Abyan province, about 120 km (75 miles) from the southern port city of Aden.
The group seized control of a significant amount of territory in Abyan during the turmoil that led to the replacement of President Ali Abdullah Saleh by his deputy, a deal that Saudi Arabia and Washington hope will prevent al Qaeda from getting a foothold near key oil shipping routes.
The conflict with Islamists in the south is only one of several challenges facing the new president, Abd-Rabbu Mansour Hadi, who took office vowing to fight al Qaeda, only to have more than 100 soldiers killed in a series of attacks in his first days in power.
Fifteen fighters were killed in Monday's clash with the army and five when warplanes bombed a checkpoint they were holding, officials and residents said. Four soldiers and one tribesman fighting alongside them were also killed.
A military official said the army drove the fighters away from the area around the camp. The militants said in an emailed statement that none of their fighters was killed in the clash, and threatened to attack Lawdar.
Mohammed Nasser, a resident of Lawdar, speaking by telephone with the sound of artillery and small arms fire audible, said the fighting lasted three hours.
"It is not the first attempt (by the group) to take control, but it's the biggest attack yet," he said. A local official said tribal militiamen joined the fighting alongside the military, and that at least 10 soldiers and tribesmen were wounded.
Washington, which has pursued a campaign of assassination by drone and missile against alleged al Qaeda targets in Yemen, wants Hadi to reunify a military that split between Saleh's foes and allies last year, and focus it on "counter-terrorism".
Yemen's main airport in the capital, Sanaa, was paralysed for a day after Hadi sacked the air force commander, a relative of Saleh, on Friday, and pro-Saleh officers responded by blockading the airport with vehicles.
A government official said they backed down only after warnings from the United States and the Gulf countries which crafted the deal that made Hadi president. (Reporting by Mohammed Mukhashaf; Writing by Joseph Logan and Mahmoud Habboush; editing by Tim Pearce)