JAKARTA (Reuters) - Indonesian President Joko Widodo called late on Monday for strong action against anyone caught lighting fires to clear forested land, as a worsening haze blanketed the north of the country and neighboring Singapore and Malaysia.

Southeast Asia has suffered for years from annual bouts of smog caused by slash-and-burn practices in Indonesia's Sumatra and Kalimantan islands, but governments in the region have failed to address the problem.

Air quality dipped to "unhealthy levels" in Malaysia and Singapore this week. Schools were closed in several Malaysian states and some flights were disrupted on Tuesday due to poor visibility. The haze can cause respiratory problems, and irritate the eyes and throat.

The fires have been exacerbated this year by the effects of the El Nino weather phenomena, as a prolonged dry season in Indonesia has parched the top soil, fuelling the flames.

Widodo, who is on a state visit to the Middle East, said he had instructed security forces to accelerate efforts to extinguish the fires.

"I have asked authorities to take strict legal action against those responsible for the forest fires, including revoking their land permits," Widodo said in a statement.

Indonesian police have named over 100 people as suspects in slash-and-burn cases in Kalimantan and Sumatra, according to local media.

The smog is usually caused by palm oil and pulp and paper companies, some of which are listed in Singapore. The firms blame small-holders for the fires but have been criticized by green groups for not doing enough to stop the haze or rampant deforestation in Indonesia.

Singapore's environment minister said late on Monday that Indonesia had agreed to share names of companies causing the fires once the information had been verified.

Indonesian Forestry and Environment Minister Siti Nurbaya Bakar last week said authorities were investigating 10 firms, which could face sanctions if found violating the law.

Indonesia's Riau province declared a state of emergency this week as, according to local media, nearly 25,000 people there and on Sumatra island suffered respiratory problems. The PSI air quality index hit a "dangerous" high of 984 in the provincial capital this week, according to the national disaster management agency.

Indonesia has deployed hundreds of military troops to fight the fires and would send in additional helicopters to water-bomb the affected areas if necessary, the national disaster management agency said.