SAN'A, Yemen - Hundreds of Yemenis fled their homes for refuge as Yemeni troops shelled villages and clashed with al Qaeda gunmen in an assault on alleged hideouts of the militant group in the remote mountains of the south, witnesses and officials said on Monday.

Amid the now six-day-old campaign in the southern province of Abyan, a court in the Yemeni capital San'a sentenced an al Qaeda militant to death after convicting him of involvement in terror attacks and manufacturing explosives.

The militant, Saleh al-Shawish, shouted in court after the verdict that al Qaeda would have its revenge. "Your destruction will be by our hands, God willing," he barked, "starting in Abyan."

With U.S. help, Yemen has stepped up the fight against al Qaeda's affiliate in the country, which is believed to have several hundred fighters entrenched in mountainous, tribal regions of the country and which claimed responsibility for the Christmas Day attempt to bomb an American passenger jet over the U.S.

Over the past month, the group, called al Qaeda in the Arabian Peninsula, has intensified a campaign of assassinations against Yemeni security officers, particularly in Abyan province, and carried out a failed attempt to kill a British diplomat in the capital. In an online message last week, it announced the formation of the "Abyan-Aden Army," aimed at toppling the Yemeni government.

Since Thursday, Yemeni troops have been waging an assault on suspected al Qaeda cells in the town of Mudiyah, about 250 kilometres southeast of the capital, clashing with fighters in the city and hammering the nearby mountains with airstrikes and tanks and artillery fire. Some of the shelling has hit in the town and in nearby villages -- small collections of mud huts -- tucked into the valleys of the very rugged region.

Hundreds of residents from around 20 villages in the area have had to flee, the Yemeni Organization for Human Rights and Freedoms said in a statement Monday. At least four civilians have been killed, including one child, it said.

One 28-year-old resident who fled told The Associated Press that Mudiyah has become a "ghost town" after most residents abandoned it and most shops shut down because of clashes in the streets.

"You can see charred husks of police vehicles, and you can even see one body of the slain militants decomposed with no one to retrieve it," he said over the phone, identifying himself only by his family name al-Mashreqi.

Al Qaeda fighters in the town carry out surprise, hit-and-run attacks on government troops, sparking heavy, random responses from the military, said another resident, 32-year-old Mohammed, who fled with his seven-member family to the nearby town of Lawdar. "Street clashes are nonstop," he said. Both spoke on condition their full names not be used because of worries of problems with authorities.

Mudiyah's deputy police chief, Lt. Mohammed al-Khodr told AP that at least six militants have been killed since the beginning of the attacks on Thursday. He said one militant, Hani al-Thuriya, was arrested carrying documents showing al Qaeda plots to assassinate top security officials in Abyan.

But another local official expressed skepticism over the death toll and said it was not possible to confirm the death of the six.

The official also said that on Sunday, 18 army officers fled and sought refugee with Mudiyah tribes after their trucks came under attack by al Qaeda militants on their way to the town. The official spoke on condition of anonymity because he was not authorized to talk about details of the operations.

The assault was sparked when al Qaeda militants killed Mudiyah's security chief and then ambushed Abyan's governor as he came to inspect the site. The governor escaped, but four people -- including his brother -- were killed in the fighting.

Yemen's central government has little direct control over large stretches of the rugged, impoverished country, and it has struggled to crack down on al Qaeda fighters taking refuge there.

The militant who was sentenced to death on Monday, al-Shawish, was arrested in February and later charged with involvement in seven attacks on security facilities, training would-be suicide bombers and bomb making.

Al-Shawish, also known as Salem al-Hadrami, confessed to blowing up security checkpoints starting in 2007, according to transcripts of his interrogations. He told the state prosecutor's office that he, along with others, attacked oil facilities in the eastern province of Hadramawt using katyusha rockets.

Over the past week, several foreign embassies -- including the United States, Britain, Australia, France and South Korea -- have issued stepped up warnings to the citizens, cautioning them against travel in Yemen, and warning of the possibility of al Qaeda attacks.