* Bombing latest in a spate of attacks by Islamist militants
* Al Qaeda-linked group says responsible for bombing
* Saleh's nephew given Republican Guard post
* Group says it took one soldier captive (Adds Saleh nephew appointment, Islamist video)
By Mohammed Ghobari
SANAA, March 13 (Reuters) - At least four Yemeni soldiers were killed on Tuesday when a suicide bomber detonated a vehicle laden with explosives near a checkpoint in the south of the country, a police source said, in an attack claimed by an al Qaeda-linked group.
The bombing is the latest in a spate of attacks by Islamist militants, who have escalated their operations in Yemen's south since President Abd-Rabbu Mansour Hadi took office last month vowing to fight al Qaeda's regional wing.
At least four soldiers were wounded in the attack outside the southern city of al-Bayda, the police source told Reuters. The governor of al-Bayda province said clashes between the army and "terrorists" erupted after the explosion.
"The attacker who drove the car made it explode when it stopped at a checkpoint," the source said. "It scattered into tiny pieces, killing four soldiers instantly. Four others were taken to the hospital with critical wounds."
Militant group Ansar al-Sharia (Partisans of Islamic Law)said it was responsible for the bombing and put the number of soldiers killed at 27, in a text message purporting to come from them. It said three of its own fighters had also been killed.
The governor of al-Bayda said the dead militants included two local al Qaeda leaders.
The militant group said the attack was in revenge for recent air strikes on al-Bayda and Abyan provinces, which it blamed on U.S. drones. Washington has repeatedly used drones to target militants in Yemen.
More than 60 Islamist fighters have been killed over the past week in air strikes attributed to the United States and Yemen.
The campaign follows a series of attacks by militants on the army, the deadliest of which killed at least 110 conscripts last week.
Ansar al-Sharia also said it had taken one soldier captive, in addition to the 73 it already claims to be holding. In a statement posted on Islamist forums on Monday, the militants said they would give free passage to the 73 in exchange for the release of their imprisoned fellow Islamists.
In a separate posting, a video showed a local leader of Ansar al-Sharia in Abyan speaking to a group of more than 50 men it identified as the captive soldiers.
"Don't you want the law of God?," asked Jalal al-Marqishi to a chorus of "yes" from the captives, who sat cross-legged on the floor beneath him. "Today we are occupied and you are occupied... Americans hold the Yemeni security portfolio today."
Wary of al Qaeda's presence in Yemen, Washington 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 protests against him.
That plan has been denounced as a U.S. and Saudi ploy to get rid of Saleh in a sop to protesters calling for his overthrow, whilst keeping his regime in place as a perceived bulwark against al Qaeda.
Those suspicions are likely to be fanned by the appointment on Tuesday of Saleh's nephew Tareq as commander of the Republican Guard's third brigade. He was formerly head of the Private Presidential Guards.
The move, confirmed by a government source, will also raise doubts about Hadi's ability to stand up to Saleh's family members and allies -- many of whom still hold senior roles in the military and security services -- and oversee a restructuring of the army, as outlined in the Gulf initiative.
Political upheaval that paralysed the impoverished nation for most of 2011 have severely weakened central government control over the country, particularly in the south, where militants have seized several towns.
Opponents of Saleh accuse him of manipulating -- even encouraging -- militancy to secure the backing of Saudi Arabia and the United States, both of which have been targeted by al Qaeda's Yemen-based wing. (Reporting by Mohammed Ghobari, writing by Sami Aboudi and Isabel Coles; Editing by Robert Woodward and Karolina Tagaris)