SANAA, Yemen — Hundreds of Islamic militants cemented control over a town in southern Yemen on Sunday, even seizing army tanks, military officials said, while breakaway army units encouraged other military forces to switch loyalties and join the uprising.

The growing number of defections in the military posed the most serious threat yet to President Ali Abdullah Saleh’s three-decade grip on his country.

A leader of the breakaway forces, Maj. Gen. Abdullah Ali Elewa, appealed to other units to join.

“Stand side by side with the courageous armed forces, Republican Guards, and security officers who endorsed the peaceful popular youthful revolution and announced their support to stand up to the tyrants and corrupt, and unjust,” he said.

General Elewa, a former defense minister, was one of nine military officers who signed the statement, named “Statement Number One” in the style of a military regime, though the officers are not in power.

The group included leaders of four of Yemen’s five military divisions.

Mr. Saleh labeled them “traitors” and “war mongers.”

“We understand the demands of the youth revolution, but we ask them first to get rid of those corrupt, agent, and traitor elements who defected from the military,” Mr. Saleh said.

Sunday night, several explosions were heard in the capital, Sanaa.

Their origin was not known.

In the southern city of Taiz, witnesses said security forces attacked protesters camped out in the central square.

Throughout the three months of protests against his 33-year rule, Mr. Saleh has repeatedly argued that al-Qaeda would overrun the impoverished corner of the Arabian peninsula if he falls.

Hundreds of Islamic militants stormed Zinjibar, a town of more than 20,000 people near the shores of the Gulf of Aden, on Friday, seizing a number of banks and government offices.

Military officials said the militants extended their control starting late Saturday night, capturing six army tanks and several armored cars after the governor, the security chief, and the commander of a local army brigade withdrew.

Other army units clashed with the militants around the town Sunday , shelling it from the outskirts and forcing terrified residents to flee or cower in their homes.

Medical and military officials said eight civilians and 18 soldiers have been killed since Saturday. Scores more have been injured.

The officials spoke on condition of anonymity.

First Published May 30, 2011, 4:30 a.m.