JAKARTA, Indonesia (AP) — Heavy smoke from illegal fires set to clear land for plantations has blanketed parts of Indonesia's Sumatra island, disrupting flights and hampering search efforts for the missing Malaysia Airlines jetliner, officials and a pilot said Thursday.

Forest and brush fires have raged for the past week, mostly in peat-rich Riau province, forcing schools to close. Children and the elderly have filled local clinics and hospitals, complaining of respiratory problems.

Forty-six hotspots were detected by satellite Thursday across Riau province, down sharply from 168 on Wednesday, said Raffles Brotestes Panjaitan, the Forestry Ministry's director of Forest Fire Control. As much as 13,000 hectares (32,123 acres) of land in Riau has burned since early last month, according to government estimates.

Panjaitan said a thick haze was covering Pekanbaru, the provincial capital of Riau and the nearest city to the fires, sending a cloud of smoke to the neighboring provinces of Jambi and West Sumatra. Visibility was reduced to less than 50 meters (yards) in some districts.

"The government has tried to halt the burning, but it has gotten out of control," Panjaitan said.

He said police had arrested at least 39 farmers for setting the fires. He did not say whether the farmers were employed by large companies that have been accused of deliberately starting the blazes.

The smoke has left more than 45,000 people with respiratory problems and eye irritation, said Zainal Arifin, who heads Riau province's health office.

Provincial authorities urged people to stay indoors due to "hazardous" levels of pollution.

State-run Garuda Indonesia and 15 other airlines have canceled flights to and out of Pekanbaru, airport official Ibnu Hasan said.

The smoke was also hampering Indonesian navy pilots searching for the missing Malaysia Airlines jetliner over the Malacca Strait as part of multinational search efforts, said Maj. Laksono, a Cassa C-212 pilot involved in the search.

"We had to fly below 500 feet (152 meters) to avoid clouds of smoke," said Laksono, who like many Indonesians uses one name. "But overall, the search went smoothly."

Indonesia's central government has sent planes and deployed more than 2,500 soldiers, police and rescuers to help douse the fires, said National Disaster Mitigation Agency spokesman Sutopo Purwo Nugroho.

Indonesia banned the practice of open burning in 1999 after widespread blazes sent thick haze over Indonesia, Malaysia, Brunei, Singapore and other nearby countries, sparking disputes with Indonesia's neighbors. However, enforcement of the law is often lax as corrupt officials turn a blind eye.

The haze is at its worst during the dry season, which runs from March to September.