KUALA LUMPUR — Malaysian Prime Minister Mahathir Mohamad is writing to Indonesia's leader to raise his concern about cross-border haze, Malaysia's environment minister said on Thursday, as a row over smoke from forest fires simmers.

Fires have burnt through parts of Indonesia's Sumatra and Borneo islands for more than a month, and the Indonesian government has sent thousands of security personnel to try to douse the blazes. They are usually set during operations to clear land for palm oil and pulp plantations.

In what has become an almost annual occurrence, especially in dry years, Indonesia's neighbors are becoming alarmed by the thick haze wafting in, and raising concern about health and the impact on tourism.

But Indonesian officials caused further anger in Malaysia this week by disputing reports that the smoke was coming from their country.

"I have discussed this with the prime minister and he has agreed to write a letter to President Jokowi to draw his attention towards the issue of trans-boundary haze," Malaysia's environment minister, Yeo Bee Yin, told reporters, referring to Indonesian President Joko Widodo by his nickname.

Mahathir's office was preparing the letter and it would be delivered soon, Yeo said.

Indonesia's forestry minister, Siti Nurbaya Bakar, said on Wednesday the problem should be viewed "more objectively" and the smoke could have originated from fires in Malaysia.

In response, Yeo said Malaysia's data was drawn from the ASEAN Specialized Meteorological Center, a Singapore-based weather station that tracks forest fire "hotspots" throughout the region.

Just five hotspots were detected in Malaysia on Thursday, compared with more than 1,500 in Indonesia, Yeo said. "The data clearly shows that the haze is from Indonesia," she said.

Malaysia closed hundreds of schools and sent half a million face masks to its Borneo island state of Sarawak this week, after the smoke built up to unhealthy levels.

The government had also prepared aircraft for cloud seeding in the hope of generating rain, Yeo said.

Several parts of Southeast Asia have endured unusually dry conditions in recent months including Indonesia, which has seen very little rain because of an El Nino weather pattern, its meteorological department has said.

Thousands of Indonesians prayed for rain in haze-hit towns on the islands of Sumatra and Borneo on Wednesday. — Reuters