The U.S. has been providing the PKK's Syrian offshoot the Democratic Union Party (PYD) and its armed wing the People's Protection Units (YPG) with weapons and ammunition that pose a threat to Turkey's national security.The U.S. air force dropped 50 tons of weapons, ammo and grenades to the YPG in northern Syria's Hasakah province in October 2015 and 112 pallets reportedly contained ammunition for M-16s and AK-47s. The Pentagon had also confirmed the airdrop."The aircraft delivery includes small arms ammunition to resupply the local forces to enable them to continue operations against ISIS [Daesh]," Pentagon spokeswoman Elissa Smith said.The YPG is believed to have a large amount of weapons and ammunition also supplied by the U.S.Meanwhile, Turkish security forces, for the first time have seized a surface-to-air missile launcher that was in the PKK's possession, providing clear evidence of the organization's expanding arsenal.The METIS M1, used for firing anti-tank missiles and surface-to-air missiles, was found in counterterror operations against the PKK in southeastern Turkey in February. Later, images of the weapon along with anti-tank missiles and a large number of munitions were shown to the press.The weapon has been in use since 1992 and was recently upgraded and used by the Assad regime in the Syrian civil war. It is believed that terrorist groups in Syria gained access to the weapon after looting Syrian army depots.Another anti-tank missile LAW was also recovered in a counterterror operation against the PKK in the southeastern province of Hakkari for the second time in early April.The missiles had been given to the YPG by the U.S. administration, media reports claimed.A huge bomb attack in the southeast Turkey earlier this month was carried out using explosives supplied by the U.S. to the PKK/PYD in Syria, a security official said late Tuesday.Speaking on the condition of anonymity due to restrictions on talking to the media, the official said the April 11 attack on a police compound in Diyarbakir was conducted using U.S.-supplied munitions.Three people were killed in the attack, for which the PKK claimed responsibility. At the time, the local governor said 1 ton of explosives had been placed in a tunnel under the complex.The anonymous official, who did not provide any further detail about the explosives or explain how the security services came to identify their origin, also claimed that weapons supplied to the PKK/PYD by the U.S. had also been found in PKK camps.Previously, another anti-tank missile had been found on a PKK terrorist who was killed in an operation in the Biztepe neighborhood of the Şemdinli district.Authorities said that they also found 500 kilograms of homemade explosives during operations in two rural districts of Nusaybin, a town in southeastern Turkey.The terrorist organization previously made unconfirmed claims that it downed a Turkish military helicopter last year.The PKK, which claims to fight for Kurdish self-rule in Turkey's southeast, has targeted security forces and civilians in the region for decades. The terrorist organization has killed thousands of people since it was established in the early 1980s.Ankara considers the YPG a terror group and the Syrian offshoot of the PKK, which is listed as a terrorist organization by the U.S., Turkey and the EU.Turkey argues that there was no difference between the YPG and the PKK, as they share the same leadership, ideology and organizational links.