At least 35 Yemeni soldiers were killed in twin suicide bombings and ensuing clashes with al Qaeda-linked fighters on Sunday, medical sources said, continuing a wave of attacks launched since a new president took office vowing to fight the group.

The campaign against al Qaeda is a key demand of Yemen's new leader by Washington, which backed his succession and has waged its own campaign of assassinations by drone strikes against alleged members of the group.

A Yemeni army officer said at least 20 of the Islamist militants also died in the fighting in the country's south, an unstable territory near oil shipping routes through the Red Sea.

Residents and local officials said cars blew up near military positions at the southern and western entrances to the city of Zinjibar, near the Gulf of Aden.

The Yemeni army sent reinforcements to Zinjibar from the nearby port city of Aden after the blasts.

Medics at a military hospital in Aden said the bodies of 35 soldiers had been brought in, and dozens more had been wounded. They said the number of casualties was likely to rise.

The attacks underscore the challenges facing President Abd-Rabbu Mansour Hadi as he tries to stabilise Yemen after a year of protests against his predecessor, Ali Abdullah Saleh, and military infighting that took Yemen to the brink of civil war.

Months of anti-Saleh protests weakened central government control over whole swathes of Yemen to the benefit of militants linked to al Qaeda, principally a group called Ansar al-Sharia, which has expanded its foothold in the south.

A text message purportedly from the group said it used car bombs to start Sunday's attacks. Ansar al-Sharia said it killed more than 50 soldiers, captured dozens of others, and seized weapons and equipment including a tank and an anti-aircraft gun.

Zinjibar has been the site of regular clashes between the army and Islamist fighters who took the city for several months last year. The government said in September it had liberated Zinjibar from militant hands, but fighting has continued.

The heroes of the armed forces have dealt a painful blow to the al Qaeda elements in ... Abyan, the defence ministry said in a text message, referring to the province where Zinjibar is located.

Residents of Jaar, which lies about 15 km (10 miles) north of Zinjibar and is controlled by Islamist fighters, said they used megaphones to urge people to join the battle.

ULTIMATUM

Last week Ansar al-Sharia said it would unleash a torrent of attacks unless the army pulled its forces away from Zinjibar within 10 days.

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

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

A U.S. ally in the fight against al Qaeda, Yemen has allowed Washington to launch drone strikes on militants who regrouped there after successive blows suffered in Iraq and Saudi Arabia.

Yemenis opposed to Saleh have pointed to the ease with which the group spread its footprint as evidence that the former president ceded it territory to amplify the al Qaeda threat and bolster his status as a key to U.S. counter-terrorism plans.

Wary of al Qaeda empowerment in Yemen, the United States and Saudi Arabia backed a Gulf-brokered plan under which Saleh gave way to Hadi, who is to oversee restructuring of the military. He appointed a new army commander in the south on Friday.

Sunday's was the second attack on as many days on military targets in the south, and one of at least five the group has claimed since Hadi was sworn in on February 25.

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

On Saturday, two suicide bombers drove a car bomb into an army base in the southern province of al-Bayda, killing one soldier. The group has also claimed responsibility for an attack on a U.S. security team in Aden on Thursday.

(Additional reporting by Mohammed Ghobari in Sanaa; Writing by Sami Aboudi; Editing by Andrew Heavens and Ben Harding)