Suicide bombers linked to al Qaeda killed at least seven soldiers in coordinated attacks on military outposts in southern Yemen Sunday, officials and residents said, part of an escalation of violence since a presidential vote two weeks ago.

The attacks underscore the challenges facing President Abd-Rabbu Mansour Hadi as he tries to restore stability to Yemen after a year of protests against his predecessor, Ali Abdullah Saleh, that pushed the country to the brink of civil war.

Residents and local officials said a vehicle containing explosives blew up at a military position held by coastal guards at the western entrance of Zinjibar, capital of the Abyan province.

Another vehicle was detonated at an artillery position at the southern entrance of Zinjibar, killing and wounding an unknown number of people.

Medical sources said at least seven had been killed and 12 were injured. Residents and local officials put the death toll at 15.

The officials said that militants, believed to be from Ansar al-Sharia - a group linked to al Qaeda that controls Zinjibar - attacked the two posts with automatic fire after the blasts, seizing equipment and capturing dozens of soldiers.

Residents of the town of Jaar, a militant stronghold located some 15 km (10 miles) north of Zinjibar, reported seeing military vehicles arriving in the town.

Months of weakened central government control have been exploited by a regional wing of al Qaeda, which has expanded its foothold in the south of the country near oil shipping routes through the Red Sea.

Sunday's attacks were among the largest by al Qaeda since the group seized parts of southern Yemen last year, exploiting popular protests against Saleh's 33-year rule.

This is a clear escalation in al Qaeda operations in southern Yemen, which comes barely a week after the new president took office, said Radwan Mohammed, an analyst based in Aden.

A Yemeni government official said the attacks were part of a campaign to create confusion for the new president.

Yemen has long been a key U.S. ally and has allowed Washington to attack al Qaeda militants who had regrouped in the rugged country after successive blows suffered in Iraq and in Saudi Arabia.

Wary of al Qaeda entrenchment in Yemen and seeking to promote stability, the United States and Saudi Arabia have backed a Gulf-brokered plan under which Saleh handed power to Hadi.

Yemenis said at least eight attacks have taken place since Hadi was sworn in last week, following the February 21 election organised as part of the power transfer deal.

The deadliest attack came hours after Hadi's inauguration, when a suicide bombing at a presidential palace in eastern Yemen killed at least 26 people.

Saturday, two suicide bombers drove a car packed with explosives into a Yemeni army base in the southern province of al-Bayda, killing one soldier. A day earlier, a Yemeni Islamist group linked to al Qaeda attacked a U.S. security team in Aden.

Just hours before Sunday's blasts, the military commander of the southern region, General Hadi Maqwala, had handed over power to another general. Maqwala had been sacked by the new president in what appears to be a move aimed at replacing some of Saleh loyalists.

(Additional reporting by Mohammed Ghobari in Sanaa; Writing by Sami Aboudi; Editing by Alessandra Rizzo)