SANAA, Yemen - About 50 Yemeni soldiers are missing after battling Islamic militants in the south of the country, a military official said Saturday.

The soldiers have been missing since Thursday, following fierce clashes with the al-Qaida-linked group Ansar al-Sharia in the southern city of Zinjibar, said the official, who spoke on condition of anonymity because he was not authorized to speak to reporters. On Thursday, 15 soldiers and eight Islamic militants were killed in fighting in Zinjibar, the official said.

He said he had no further information on the fate of the missing soldiers.

Elsewhere in Yemen, gunmen blew up an unused oil pipeline in the central province of Marib, the latest in a series of attacks on the same target in recent weeks, officials said Saturday. The attack occurred Thursday, they said.

Yemeni authorities stopped producing oil in May because of repeated attacks and labour unrest.

Yemen's growing turmoil, including the government's battle against al-Qaida's most dangerous wing in the south of the country, come at a time when the weakened regime of President Ali Abdullah Saleh faces an array of opponents. Government forces do not appear to have the will to fight the Islamists, raising fears that al-Qaida is making significant gains.

Recent advances made by the militants in the increasingly lawless south are a clear attempt to exploit the power vacuum and turmoil caused by a popular uprising against Saleh that began in February. The revolt gained momentum when a coterie of the president's close aides, military commanders and Cabinet ministers joined the protesters.

Government troops and warplanes have so far targeted only two southern cities, Zinjibar and Jaar, in Abyan province.

Yemen's president, meanwhile, remains in Saudi Arabia, where he is being treated for injuries sustained in a June 3 attack on his palace in the Yemeni capital of Sanaa. Saleh suffered serious burns and other wounds.

Yemeni officials said Saturday that after undergoing two surgeries, Saleh remains bedridden and has trouble breathing and talking. Only relatives and his top adviser are allowed to visit him, one official said.

Earlier this week, a Yemeni TV network sent a crew to the Saudi capital to record an audio message from Saleh to the Yemeni people, but authorities prevented them from entering the hospital, a Yemeni official in Riyadh said.

"They were only allowed to film the hospital from outside," the official said, citing an example of the Saudi restrictions on Saleh's visitors.

Over the past week, Yemeni ruling party officials in Sanaa have suggested Saleh may deliver an audio message on state TV to assure his people. However, a week passed without word from Saleh.

"If he (Saleh) delivered a speech through an audio message, people would not believe it is him because they will not recognize his voice," said another official, adding that Saleh's voice box was harmed.

All three officials spoke on condition of anonymity because of the sensitive nature of the issue.

The president has not been seen in public since the attack. On June 5, hours before he flew to Saudi Arabia, he aired a brief audio message, blaming an "armed gang of outlaws" for the attack on his palace.

Saudi Arabia has been pressing Saleh to step down within 30 days and hand power to his vice-president, in exchange for immunity from prosecution. A national unity government would run the country until elections are held, according to the deal brokered by the six-nation Gulf Cooperation Council.

The United States, in favour of peaceful power transfer, fears that al-Qaida's branch in Yemen could further exploit Yemen's turmoil to strengthen its presence there. Al-Qaida-linked groups have already used Yemen as a base for plotting two attempted anti-U.S. attacks.