DIYARBAKIR, (Reuters) – Turkish security forces backed by helicopters seized an estimated $22.5 million worth of marijuana along with guns and fertilizer in a swoop against suspected Kurdish militants on Saturday, officials said.

Hundreds of soldiers, police and special forces carried out the coordinated raids in seven villages around the southeastern city of Diyarbakir, seizing drugs, guns, ammunition and ammonium nitrate, which can be used to make explosives, officials said.

Turkish officials say the drug trade is a major source of funding for Kurdistan Workers Party (PKK) militants, who since 1984 have been fighting to carve out a Kurdish state in Turkey’s southeastern border region with Iran and Iraq.

The PKK uses the remote Kandil mountains in northern Iraq as a base from which to stage attacks on Turkish territory. The group is listed as a terrorist organization by Turkey, the United States and the European Union.

“The income obtained from drugs is sent to Kandil … The organization is earning a significant income though these drugs,” Diyarbakir Governor Mustafa Toprak told reporters.

The PKK has mainly carried out guerrilla-style attacks against Turkish military targets in the southeast but has also killed civilians with bomb attacks in major cities including Istanbul, Ankara and Izmir.

Fighting flared over the summer and the security forces have stepped up operations against suspected PKK bases in recent weeks out of fear that they could try to stage further attacks in the winter months.

The military carried out several raids on suspected PKK bases in the Bestler-Dereler area of the neighboring province of Sirnak last week, destroying 32 of the group’s camps and seizing explosives, the governor’s office said on Saturday.

There have also been raids by the security forces against two PKK groups in the province of Tunceli further north, security sources said.

Toprak said the security forces had seized 21 tonnes of marijuana and destroyed drug plantations in the fields around the villages in Saturday’s operation. Three people were arrested while a fourth was still being sought, he said.

More than 40,000 people have been killed in 28 years of fighting between Turkey and the PKK, a conflict which has hampered development in one of the country’s poorest corners.

In 2009, U.S. authorities named several senior members of the PKK as suspected narcotics traffickers, freezing their assets and banning U.S. citizens from doing business with them.

Turkey is an important route for drug trafficking to Europe both as a producer and an importer, mainly from Syria and Iran.

Turkey seized around 47 tonnes of marijuana in 2011 as a whole, a 50 percent rise on the previous year, according to Turkish police figures.