MANILA- Two Australian surveillance aircraft began flying over Marawi City on Friday as part of Australia's technical support to Philippine troops pursuing offensives against Islamic State-linked terror groups in Marawi City.

Defense Secretary Delfin Lorenzana said the assistance was "valuable" and that Filipino pilots and technicians were aboard the aircraft and have been relaying information to the military headquarters.

"[The mission] started last Friday and they will be here for two weeks. They are now flying alternately. There are two of them and flying alternately," Lorenzana told reporters in Camp Aguinaldo.

"We have Filipino pilots and technicians aboard and they are the ones relaying signals on the ground, to us here at the GHQ (general headquarters) and Zamboanga [City]," he said.

Lorenzana said the two Australian aircraft were flying over Marawi City even at night time, as they have the ability to fly for 24 hours.

"Then our people in Zamboanga and here (in GHQ) transmit the images taken by the P3 Orion to our troops in Marawi. That's how it works... We have people up there so that if there are important information that should be transmitted, they can do it immediately," he said.

He said Filipino troops stand to "benefit from enhanced airborne surveillance of the area anytime of the day, thereby improving operations on the ground."

On June 23, the defense department announced President Duterte's acceptance of Australia's technical assistance.

The Philippine and Australian governments have standing defense relations, having signed a Status of Visiting Forces Agreement in 2007, ratified by the Philippine Senate in 2012.

The United States military also deployed a surveillance aircraft to Marawi City on June 10 as part of technical assistance to Philippine troops.

America's support came despite President Duterte's averse stance towards the United States, as he pursued a foreign policy pivot away from traditional allies.

Duterte accepted the need for US assistance in the Marawi operations despite initially denying seeking help from the Americans.

More than a month since clashes began, government troops are still pursuing terrorists lurking around Marawi City. The death toll has reached 461, with 337 terrorists, 85 government forces, and 39 civilians among those killed.