Manila: Rescuers struggled on Sunday to reach the slopes of an active volcano in the central Philippines where a small plane is believed to have crashed, officials said, with the fate of those on board still unconfirmed.

The Cessna 340 aircraft with four on board went missing shortly after departing for Manila from Bicol International Airport in Albay province, several kilometres from Mayon volcano, on Saturday.

Manila-based Energy Development Corporation said the missing plane belonged to the company and it was seeking to confirm if the wreckage spotted on Sunday in "very steep terrain and high altitude of about 6,000 feet" was theirs.

Two of the four passengers on board the missing plane were Australian, Bicol police said, adding rescue teams with sniffer dogs had been deployed to find the crash site.

"The problem is the weather is bad and it hampers the visibility of the ground search," Civil Aviation Authority of the Philippines spokesman Eric Apolonio told AFP.

Albay disaster officer Cedric Daep said a Cessna aircraft had been seen 300 to 350 metres (984 feet to 1,148 feet) from the crater. But he warned the rumbling volcano could erupt at any time, which was complicating rescue efforts.

"There might be a sudden ash explosion and we could be added to the casualties," Daep told local radio DZBB.

An aerial search will continue Monday to locate the missing people.

"We are not discounting the possibility that they could still be alive," Daep said.

Another Cessna plane went missing last month, on January 24, in the northern province of Isabela. That plane's wreckage has still not been found, Apolonio said.

In a separate incident, two Philippine air force aviators were killed in a training exercise last month when their SF260 Marchetti plane crashed into a rice paddy in Bataan province, near Manila.