At least eight fighters linked to al Qaeda were killed in Yemen on Thursday, officials and tribal sources said, as a government offensive against Islamist insurgents entered its fourth day.

Residents and officials said three strikes by Yemeni warplanes on areas controlled by militant group Ansar al-Sharia killed at least six fighters near the southern town of Lawdar.

One strike hit an army tank that had been captured by the militants on Monday, killing everyone inside it, the sources said. Two other air strikes were on posts held by militants but no casualties were reported.

Two militants were also killed in clashes with army-allied tribesmen who tried to stop a group of armed men from Ansar al-Sharia joining forces with militants in Lawdar, said an official in the city of Mudiyah, 15 km (10 miles) to the east. One tribesman died in the fighting.

Reeling from a year of political upheaval that eventually unseated former president Ali Abdullah Saleh, Yemen is grappling with militants who have exploited weakened central government control to expand their influence, particularly in the south.

At least 150 people have been killed in fighting and air strikes in the Lawdar area since Monday when Ansar al-Sharia attacked a military camp there, according to officials.

Ansar al-Sharia said in an e-mailed statement on Wednesday only seven of its fighters had died in those clashes.

Another four militants, among them a foreigner, were killed in shelling by the army on the southern city of Zinjibar late on Wednesday, a local official said on Thursday.

A spokesman claiming to belong to Ansar al-Sharia confirmed the report and told Reuters in a telephone call that the foreigner was a Tunisian national.

(Reporting by Mohammed Mukhashaf; Writing by Rania El Gamal; Editing by Andrew Roche)