Indonesian President Susilo Bambang Yudhoyono has apologised to Singapore and Malaysia over fires that have cloaked the countries in thick haze, as thousands of emergency workers were deployed Tuesday to tackle the blazes.

Southeast Asia's worst smog crisis for years pushed haze levels in Singapore to a record high last week, with residential buildings and skyscrapers shrouded and daily life for millions in the city-state dramatically affected.

The smog has drifted north and is now badly affecting Malaysia, while in a badly-hit province on Indonesia's Sumatra island -- where the fires are raging in peatland -- hundreds gathered to pray for rain.

The crisis has triggered a war of words between Jakarta and its neighbours, with an Indonesian minister accusing Singapore of acting "like a child". But Yudhoyono sought to ease tensions by issuing a public apology late Monday.

"As the president of Indonesia, I apologise for what has happened and ask for the understanding of the people of Malaysia and Singapore," he said. "We accept it is our responsibility to tackle the problem."

Prime Minister Lee Hsien Loong said Singapore accepted Yudhoyono's "gracious" apology, adding: "We need a permanent solution to prevent this problem from recurring annually."

Indonesia had previously sought to deflect blame for the crisis, saying Singaporean and Malaysian companies who own plantations on Sumatra were also responsible.

But Malaysian Prime Minister Najib Razak said late Monday the question of who owns the plantations "is not the issue here" and called on Jakarta to take action against those responsible, national news agency Bernama reported.

Police in Riau province, where the fires are centred, said they had arrested nine people so far on suspicion of starting the blazes, all small palm oil farmers.

Smog from Sumatra is a recurring problem during the June-September dry season, when big companies and smallholders alike light fires to clear land, in a cheap but illegal method of clearing space for planting.

Several big palm oil companies have been accused of lighting fires on their concessions in Sumatra, and the Roundtable on Sustainable Palm Oil (RSPO) said Tuesday it would investigate five of its members over the allegations.

The RSPO, which produces a sought-after certification for producers deemed sustainable, bans its members from using burning to clear land.

Southeast Asia suffered its worst smog outbreak in 1997-98, which cost the region an estimated $9 billion, and was hit with a serious recurrence in 2006.

Indonesia's national disaster agency said Tuesday that more than 3,000 personnel -- including members of the army, air force and police -- would be sent over the next two days to Riau to join some 2,300 already tackling the blazes.

Firefighters are backed by helicopters and planes dropping water and attempting to chemically induce rain through cloud-seeding.

After efforts in previous days proved ineffective, cloud-seeding managed to successfully induce rains in several parts of Riau on Tuesday, officials said.

While the smog has lifted from Singapore, which was enjoying its third straight sunny day on Tuesday after the air pollution index eased from the all-time highs of last week, Malaysia is now bearing the brunt of the crisis.

Air quality was "hazardous" in two Malaysian districts, including the country's busiest port, Port Klang on the Strait of Malacca facing Sumatra, where the readings stood at 484 mid-morning Tuesday.

Readings above 300 indicate "hazardous" conditions. Three other areas, mostly in central Malaysia near the capital Kuala Lumpur, logged "very unhealthy" air quality.

In one Riau district almost 300 people were evacuated over the past two days as fires raged close to their houses, and in the province's badly-hit Dumai city hundreds gathered to pray for rain.

"This morning, we prayed to God for rain and for the efforts to fight the haze to be successful," said local environment official Basri, who like many Indonesians goes by one name.

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