* Militants seize southern checkpoint, retreat with tanks
* Qaeda-linked group claims responsibility, says killed 30
* Officials say seven militants killed in air strike, clashes (Updates death toll)
By Mohammed Mukhashaf
ADEN, March 31 (Reuters) - Islamist militants stormed a military checkpoint in southern Yemen early on Saturday, killing at least 20 soldiers, a senior military official said, the latest in a spate of attacks claimed by an al Qaeda-linked group.
Local officials said heavy fighting broke out between the militants and army reinforcements sent from the port city of Aden to retake the checkpoint, located on the road linking the southern provinces of Lahej and Abyan, which was still closed off on Saturday evening.
Yemeni war planes then bombarded the site, forcing some of the militants to retreat towards their main stronghold in the city of Jaar, taking with them two tanks and other hardware.
Emboldened by a year of political upheaval in Yemen, Islamist militants have stepped up their attacks on the army since President Abd-Rabbu Mansour Hadi took office last month vowing to fight al Qaeda's regional wing.
In their deadliest attack yet, militants killed at least 110 soldiers and took dozens hostage earlier this month in the Abyan provincial capital, Zinjibar. That followed a suicide bomb that killed some 26 people on the day Hadi was sworn in as president in February.
The government has responded with air strikes on suspected Islamist hideouts, and the United States has repeatedly used its drones to attack militants, who have seized several southern towns over the past year.
The militant group Ansar al-Sharia (Partisans of Islamic Law) claimed responsibility for Saturday's attack. In a text message purporting to come from the group, it said it had killed 30 conscripts.
"The holy warriors of Ansar al-Sharia this morning carried out the raid of dignity on the al-Hurur military checkpoint in Abyan, resulting in the deaths of around 30," read the message, whose authenticity could not immediately be verified.
The senior army officer said the militants had crept up on the checkpoint at dawn, when the soldiers were still sleeping, taking the guards by surprise and killing them first.
Three Islamist fighters were killed in an air strike on one of the tanks they had seized and four died in the clashes at the checkpoint, local and army officials said.
Residents said the army had begun distributing machineguns among them so they could help beat back the militants.
An officer in the security forces survived an assassination attempt in the city of Mukalla in Hadramout province on Saturday when a bomb planted in his car exploded just after he got out, a security official said.
Separately, unidentified gunmen riding a motorcycle opened fire on the deputy head of Lahej province's security services, wounding him, an official said.
Washington, wary of al Qaeda's growing strength in Yemen, backed Hadi's election last month under an Arab Gulf-brokered deal to ease his predecessor Ali Abdullah Saleh from power after a year of demonstrations against him.
That deal has been denounced as a U.S. and Saudi ploy to get rid of Saleh in a sop to protesters calling for his overthrow, while keeping his regime in place as a perceived bulwark against al Qaeda. (Additional reporting by Mohammed Ghobari; Writing by Isabel Coles; Editing by Tim Pearce)