MANILA, OCT. 5 (FRIDAY) -- The government launched an attack today on an army headquarters seized by rebels, who had declared a separate republic on the island of Mindanao, according to military sources in the capital.

The sources said two air force planes -- World War II-vintage T-28s or "Tora-Toras" -- fired 28 rockets and about 1,000 rounds from .50-caliber machine guns at the rebel-held headquarters of the 402nd Infantry Brigade at Butuan city on Mindanao. The army installation was one of two seized by rebels Thursday.

The government had no reports of casualties or damage from the attack, although a Manila radio station said several buildings had been set on fire.

The sources said the attack was a prelude to a larger assault today on the main rebel stronghold at Cagayan de Oro city, where the rebel leader, Col. Alexander Noble, was holed up with most of his followers. F-5 jet fighters were "warming up" at an air base near Manila for an attack on the mutineers, one source said at 1 p.m. today, and a Marine brigade was "landing" in Cagayan de Oro.

On Thursday, the rebels, led by Noble, a former deputy commander of President Corazon Aquino's guard, declared a separate republic on Mindanao, the Philippines' second-largest island. Military officials said the revolt was part of a large plot to launch uprisings elsewhere in an attempt to oust Aquino and take over the government.

The armed forces were put on nationwide alert, and Aquino issued a radio appeal for support Thursday. She broadcast a list of telephone numbers for Filipinos to call if they have "any information to share with the government" about plots by the rebels. There has been no sign of support for the rebels in other parts of the country.

Early this morning, a spokesman for Aquino said the rebellion had been contained. "We were monitoring it overnight. It's really in isolation," spokesman Tomas Gomez told Reuter.

Defense Secretary Fidel Ramos, who cut short a visit to South Korea and returned to Manila Thursday, said all governors and mayors in Mindanao's 22 provinces had pledged loyalty to the government and warned that civilians supporting the mutineers would be charged with rebellion.

"We will establish control very soon," he said. "The important thing is the support of the people."

Noble, who participated in a coup attempt last December that left scores dead and hundreds wounded, marched into Cagayan de Oro in Misamis Oriental Province Thursday, hours after his forces took over the army brigade headquarters in Butuan, capital of the adjacent province of Agusan del Norte.

With a convoy of trucks and armored personnel carriers, Noble led a victory parade into Cagayan de Oro, attracting a large, cheering crowd. Officials said Noble, who has about 400 followers and militia forces belonging to a native tribe, the Higoanon, immediately seized Camp Evangelista, headquarters of the 4th Infantry Division, with no resistance.

"In the name of the people of Mindanao, I am now in control," declared Noble, wearing a camouflage uniform with a knife on his right hip and an amulet around his neck. The ex-colonel had been holed up in the mountains with the tribesmen.

The army reported that the rebels also assaulted an infantry battalion headquarters in a third city, Iligan, Reuter reported, but were forced to retreat by government fighter-bombers, which also destroyed a rebel helicopter with rocket fire.

In Manila, the underground military rebel movement distributed a statement to news organizations saying the mutiny had "liberated" Mindanao from the "clutches of the imperialist-dominated elitist" Aquino administration.

"They have established the free and sovereign Federal Republic of Mindanao . . . with the hope of later reuniting with a constituted Federal Republic of the Philippines," it said.

Earlier this year, Noble forged an alliance with a civilian group called the Mindanao Independence Movement, which has been seeking autonomy in the largely Moslem-dominated island of 17 million people. An insurgency on Mindanao in the 1970s killed thousands of people. Independence movement leader Ruben Canoy greeted Noble with a handshake and announced to the press that he had sent letters to foreign embassies asking recognition for the new republic.

U.S. Ambassador Nicholas Platt said the United States "strongly condemns any effort to destabilize the elected Philippine government."

{In Washington Thursday, the State Department reaffirmed its "absolutely firm" support for the Aquino government and noted that U.S. law requires an immediate cut-off of foreign aid whenever an elected government is ousted by a coup, staff writer Al Kamen reported. "This is not subject to discretion and will occur in the event of a coup," a department spokesman said. U.S. aid now is about $480 million.}

Rebel supporters dismantled roadblocks as Noble made his way into the camps. Sources said at least two army battalions defected to Noble, and the army division's commanding general, Miguel Sol, abandoned his post and disappeared. Soldiers told reporters they saw the general fleeing.

Aquino, who has survived a half-dozen previous coup attempts since she became president in 1986, called on Filipinos to "rally against these troublemakers."