Mexico City (AFP) - A special Mexican police unit has raided seven sawmills near the monarch butterfly's mountain sanctuary in a bid to prevent illegal logging threatening the insect's winter migration, officials said Tuesday.

Backed up by a helicopter, some 220 members of the country's police force and 40 forestry inspectors participated in the September 12 operation in the western state of Michoacan.

North American governments have taken steps since last year to protect the monarch butterfly, which crosses Canada and the United States each year to hibernate on the fir and pine trees of Mexico's western mountains.

Last week's raid was the first since the government decided in April to add the police to protection efforts for the brilliant orange and black monarchs.

The force has been conducting foot patrols day and night, using drones and helicopters for surveillance when weather permits, Abel Corona, director of the special units, said at a news conference.

The authorities permanently shut down three sawmills in the town of Ocampo and one was temporarily closed while its legality is verified, the government said.

Three more were temporarily closed in Aporo for lack of the proper paperwork.

No one was arrested. The operators of the illegal mills fled before the police arrived.

Ignacio Millan Tovar, a deputy prosecutor at the federal environmental prosecutor's office, said the raids led to the seizure of 6.54 cubic meters (231 cubic feet) of wood.

By permanently shutting down three mills, the authorities are preventing the illegal logging of 3,300 cubic meters of wood per year, Millan said.

"It is the equivalent of 330 logging trucks lined up one after the other," Millan said.

"This is the relevance and importance of removing these three sawmills from circulation," he said.

The people behind illegal logging are from Michoacan, he said, without providing more details.

Illegal logging dropped by 40 percent between the 2014-2015 and 2015-2016 butterfly season, environmental protection authorities said last month.

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But March storms killed seven percent of the monarchs.

The cold spell came after authorities had reported a rebound in the 2015-2016 season, with the butterfly covering 4.01 hectares (9.9 acres) of forest, more than tripling the previous year's figure.

The cold weather's impact on the population will be known when the butterflies begin to arrive in later October or early November, before returning north in March, said Gloria Tavera Alonso, regional director of CONANP, the agency overseeing natural protected areas.