JAKARTA , Indonesia (AP) - Smoke and ash from land-clearing fires in Indonesia blanketed a large swath of the country's west on Monday, forcing residents to wear protective masks, sending air quality plummeting and delaying flights, officials and media reports said.

Skies were also hazy in neighboring Singapore due to the fires, a media report there said.

The smoke was shrouding an estimated 556,000 square kilometers (215,000 square miles) of land on the western islands of Sumatra and Borneo , officials and media reports said.

"The haze has persisted for a whole week,'' said Frans Tandipau, a senior official tasked with extinguishing forest fires on Sumatra island.

Fires from land-clearing activities in Sumatra and Borneo islands, 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 early Monday when the haze was at its worse.

Air quality levels had reached "dangerous'' levels, from "unhealthy'' last week, according to a monitoring station in Palangkaraya, state news agency Antara reported.

Indonesia has been often criticized by its neighbors for not tackling the problem.

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 put the problem.