JAKARTA (AFP) - Visiting Malaysian Prime Minister Najib Razak pledged on Sunday to help Indonesia fight forest fires blanketing Southeast Asia in haze as foreign aircraft joined operations to douse the blazes.

Datuk Seri Najib began a two-day visit by meeting President Joko Widodo at the state palace in Bogor, West Java, to discuss a range of issues including the palm oil industry, environmental cooperation and the smog.

The regional environmental crisis has caused flights and major events to be cancelled and forced tens of thousands of people in Indonesia, Malaysia and Singapore to seek treatment for respiratory problems.

"We consider the haze a serious issue as it's a burden to Malaysians and Indonesians," Mr Najib told reporters after the meeting.

"Malaysia is prepared to increase our assistance in dousing the fires. The areas affected are widespread so certainly the challenges are very big," he said.

The blazes flare annually during the dry season as fires are illegally set to clear land for cultivation on Indonesia's island of Sumatra and in the Indonesian portion of Borneo island.

National disaster mitigation agency spokesman Sutopo Purwo Nugroho said a Malaysian CL415 Bombardier capable of scooping up to six tonnes of water from the sea, and a Singaporean Chinook helicopter which can pour water from a huge hanging bucket, were among the 10 aircraft used for water-bombing in South Sumatra province.

A Lockheed L100 Hercules Air Tanker with a 15-tonne capacity is expected to arrive in the coming days, he added.

Jakarta has deployed about 25,000 personnel and aircraft but the firefighters have been overwhelmed by the extent of the blazes.

The Indonesian government for weeks insisted it did not need international help before finally agreeing to accept the offers from several countries.