Next weekend's Singapore Grand Prix is reportedly in doubt due to billowing smoke from Indonesian forest fires.

The worsening air pollution conditions have alarmed F1 chiefs who may be forced to pull the plug.

Yahoo! reports that the Pollutant Standards Index, used to measure the levels of air pollution in Singapore, rose to 222 in the early hours of Monday - enough to close schools and limit working hours.

Several outdoor events have since been cancelled, but no decision has yet been made on whether the Grand Prix - which is due to get underway in six days' time - will go ahead.

"In the event that the haze caused visibility, public health or operational issues, Singapore GP would work closely with the relevant agencies before making any collective decisions regarding the event," the Singapore Grand Prix said in a statement.

Lewis Hamilton currently leads the way in the drivers' standings, sitting well ahead of Mercedes teammate Nico Rosberg who occupies second place.