A rise in clearing farmland by burning leftover straw has increased air pollution in parts of northeastern China, the environmental watchdog has warned.

The Ministry of Environmental Protection found 862 suspected hot spots nationwide from Oct 5-17 from satellite sensing technology, an annual increase of 6.7 percent, authorities said on Sunday.

The fires have been a major reason for rising air pollution in northeastern China, with the environmental watchdog pointing out that many hot spots were found in areas where burning farmland fields is illegal such as near airports and expressways.

"Among the 324 suspected spots exposed last week, 31 percent of them were found in Liaoning province," said Wang Dongqing, deputy head of the ministry's environmental monitoring department.

Apart from satellite evidence, ground inspections showed an increase in farmland fires on the outskirts of Shenyang and Tieling in Liaoning province on Friday and Saturday, the ministry reported.

"October is the main month for clearing farmland by burning fields in provinces of Liaoning, Jilin and Heilongjiang, which has a negative effect on air quality," Wang said.

The three provinces accounted for 63.2 percent of the country's farmland hot spots between Sept 20 to Nov 20 last year.

This in turn pushed Shenyang, the capital of Liaoning province, into the top 10 of cities suffering severe air pollution.

Using limited amounts of straw for domestic heating is not an option with cheap coal prices, so farmers simply burn it, Sun Baoya, head of the agricultural environmental protection department of Shenyang, told Central China Television on Monday.

The three provinces have also failed to come up with a collection process to ease the problem, Wang at the Ministry of Environmental Protection said.

zhengjinran@chinadaily.com.cn