Bangkok, Jun 21 (IANS/EFE): Air pollution in Singapore Thursday reached levels dangerous for human health due to smoke from fires set in nearby Indonesia to clear land for crop planting.

The pollution index in the city-state reached a maximum of 371 points, 71 points above the level considered "dangerous", and there has been an increase in the number of patients suffering from respiratory problems in local hospitals, media reported.

Singapore's Prime Minister, Lee Hsien Loong, asked his countrymen to remain indoors and to try to go outside as little as possible after the worst air pollution since 1997 was reported, at which time the pollution index topped out at 266.

At an index level of 200 and above, the air is considered to be "very unhealthy", according to the scale used to measure pollution.

A murky haze is presently floating between Singapore's skyscrapers and its bay, but the first clouds of smoke began to arrive last week from fires on the Indonesian island of Sumatra, given that this is the season when farmers begin clearing land so that they may plant their crops.

"The workers must wear protective masks if they perform work outdoors and if they show symptoms of feeling bad, they have the right to leave work and rest in an enclosed space," said Singapore's National Congress of Unions in a statement.

The pollution has also affected visibility in the Straits of Malacca, where boats have had to limit their operations at night, and even in Malaysia, where 211 schools were closed in the southern part of the country due to a pollution level of 383 points.

Indonesia has blamed the fires on foreign investors who own plantations in Sumatra and urged Singaporeans not to react like "children, with so much excitement" to the smoke.