Six Indonesian provinces have declared states of emergency as forest fires blanket a swath of Southeast Asia in a smoky haze.

Singapore's air quality deteriorated to unhealthy levels on Friday as winds blew smoke from fires on Sumatra across the city-state and southern Malaysia.

The number of hotspots detected by weather satellites has increased in the past month though they are below levels last year when massive fires caused a regional crisis.

Indonesia's Disaster Mitigation Agency says six provinces have declared emergency, allowing firefighting measures to go into full effect.

Many of the fires are deliberately set by agricultural conglomerates and small-time farmers to clear forests and peatland for plantations.

Last year's fires were the worst since 1997, with about 261,000 hectares (2,610 square kilometers) burned.