ASUNCION, Sept 12 (Reuters) - Paraguay's government declared a national state of emergency on Wednesday as it battles the worst forest fires ever to hit the poor South American country.

The flames have devoured more than 100,000 hectares (nearly 250,000 acres) of forest, crops and grazing pasture and burned down dozens of rural homes in three northeastern provinces, where a regional state of emergency was already in place.

President Nicanor Duarte Frutos, who has been criticized for reacting too slowly, said the state would pay to rent a Russian water-carrying aircraft that had been used to fight devastating blazes that swept Greece in August.

"With this plane, we'll be stepping up the government's work in the northern zone," he told reporters. The widening of the state of emergency was posted on a government Web site.

Officials say the fires were started by farmers clearing farmland and renewing pastures, a common practice in the country, which is a significant producer of soybeans and beef.

However, tinder-dry conditions caused the flames to spread close to some urban areas and into a forest reserve, where environmental groups say protected trees are at risk.

"Trying to contain the fires is difficult because the access is very limited," said Danilo Salas from the Moises Bertoni Foundation, which protects the Mbaracayu reserve.

Agricultural and environmental experts say the forest fires are the worst in the country's history.

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