In depth

  Country profile: Yemen
  Profile: Al-Qaeda in the Arabian Peninsula
  Interview: Anwar al-Awlaki
  Riz Khan: Yemen, a failed state?
  Video: Yemen's tough al-Qaeda challenge
  Inside Story: Yemen's future
  Inside Story: Can the West save Yemen?
  Inside Story: Yemen - An international quagmire?
  Focus: The $30bn pair of underpants
  Focus: Child soldiers used in Yemen war

The sources said the attack was in retaliation for an army raid on the home of a tribal chief, Sheikh Nasser Gammad bin Dawham, who stood accused of sheltering al-Qaeda members. They gave no details on the raid.

On June 5, a Yemeni colonel and two soldiers were killed in an attack by suspected al-Qaeda members near the city of Maarib as they travelled in convoy to inspect military forces stationed in the Safar oilfield.

Yemeni forces have been battling al-Qaeda operatives in Maarib for four days. The government say it aims to catch suspected al-Qaeda fighters believed to be behind the ambush. 

Tribesmen from Maarib, where al-Qaeda has a strong presence, last month set ablaze two oil pipelines near the Safer fields but the authorities have since repaired the damage.

Yemen is the ancestral homeland of al-Qaeda leader Osama bin Laden.

The group has suffered setbacks amid US pressure on Sanaa to crack down.