JAKARTA - Indonesian Transport Ministry has issued a warning to planes passing Indonesian territory of the danger of haze from forest fires in Sumatra island, a minister said here on Monday.

Indonesian Transport Minister E.E. Mangindaan said that his ministry already issued a warning letter to aviation firms about the danger of the haze, saying that the fire has expanded and endangered flight.

"The warning letters have been spread," he told reporters at the State Palace.

But, the minister said that so far his ministry had not barred flight.

"(The plane) must be careful. Please do not pass in if there is haze or cloud," said Mangindaan.

The haze from Riau province of Sumatra island has spread to neighboring countries of Singapore and Malaysia.

The Indonesian government is struggling to constrain the forest fires caused by land clearing for opening of new plantation.

Haze is an annual problem in Indonesia during drier summer months, when westerly monsoon winds blow smoke from forest fires and slash-and-burn land-clearing on the huge island of Sumatra, which lies across the Malacca Strait from Malaysia and Singapore.