ADEN (Reuters) - A U.S. drone attack killed at least five suspected al Qaeda militants in southern Yemen on Friday, and gunmen retaliated by blowing up an LNG pipeline, forcing output to be stopped, officials and energy workers said.

The drone set fire to the militants' car in the southern province of Shabwa and killed all its occupants, one official said. One bystander was also killed and five were wounded, officials and residents told Reuters.

Hours later gunmen believed to be militants blew up a pipeline which transports gas to a facility whose leading stakeholder is French oil major Total at Balhaf port on the Arabian Sea, energy workers said.

Residents said flames could be seen from several kilometres (miles) away and a company employee said exports had stopped.

"The explosion took place 28 km (17.5 miles) north of the Balhaf LNG export plant. Production has been halted," an employee of Yemen LNG, who asked not to be named, told Reuters.

The Balhaf LNG export facility opened in 2009 and was the largest industrial project ever carried out in impoverished Yemen.

Oil and gas pipelines have often been attacked by Islamic militants and disgruntled tribesmen. The pipeline to Balhaf was last blown up in October, hours after an air raid on militants, and took about 10 days to be repaired.

A text message sent to journalists, purporting to come from the al Qaeda-affiliated Ansar al-Sharia (Supporters of Islamic Law), said the group was behind the attack.

"The mujahideen (holy war fighters) blew up the pipeline ... in retaliation for the strike for which Crusader America and its obedient slave in Sanaa are responsible," the text message said, referring to the Yemeni government, a close U.S. ally in the fight against al Qaeda.

Al Qaeda has strengthened its hold on southern areas of the Arabian Peninsula country during the past year of protests against veteran ruler Ali Abdullah Saleh, who left office in February.

The United States has retaliated with a campaign of drone strikes on suspected al Qaeda militants.

In an earlier text message, Ansar al-Sharia said two militants were "martyred" in the attack and four passersby were injured.

Earlier this month, U.S. drone attacks killed at least 25 al Qaeda-linked fighters including one of their leaders, and a Yemeni air force raid killed 20, in the biggest airstrikes since the new president, Abd-Rabbu Mansour Hadi, took office.

Neighboring Saudi Arabia, the world's biggest oil exporter, shares U.S. concern over the expansion of al-Qaeda's regional wing in a country next to major Red Sea oil shipping lanes.

In a separate incident, suspected al Qaeda fighters shot dead two men in the southern port city of Aden on Friday, a local official said. The victims belonged to a clan that has opposed the group's militants in nearby Abyan province, he said.

In a persistent dispute over fishing rights with Eritrea, Yemeni forces captured two Eritrean soldiers trying to seize a Yemeni fishing boat near the Red Sea port of Hudaida, Yemen's Interior Ministry said.

(Reporting by Mohammed Mukhashaf; Writing by Firouz Sedarat; Editing by Tim Pearce)