Smoke and ash from land-clearing fires in Indonesia blanketed a large swath of Indonesia's west Monday, sending air quality levels plummeting there and in neighboring Singapore and Malaysia.

The smoke was shrouding an estimated 556,000 square kilometers of land on Sumatra and across Indonesian Borneo.

Fires from land-clearing activities in Sumatra and Borneo, and to a lesser extent Malaysia, have occurred almost every dry season since the late 1990s. They are typically set by people looking for a cheap way to clear brush for plantations.

In Palangkaraya, on Kalimantan on the Indonesian side of Borneo island, many people were wearing face masks, residents there said. Flights in and out of airports were delayed Monday.

Indonesian President Susilo Bambang Yudhoyono has vowed to crack down on landowners who set fires illegally, but inefficient and corrupt local authorities are apparently unable or unwilling to stamp out the problem.