According to four current and former US officials, cited by The Washington Post, Turkish forces knew for months that a US Special Operations outpost was stationed in the area.
One US official said, cited by WaPo, that multiple rounds of 155mm fire were launched from within Turkey and that they had a “bracketing effect” in which shells landed on both sides of US positions.
"That’s an area weapon [...] That’s not something we ever would have done to a partner force", the official said, adding that Turkish forces knew there were Americans on the hill and that it had to be deliberate, the DC-based newspaper reported.
According to an earlier Pentagon statement, Turkish shells "exploded a few hundred meters of a location outside the Security Mechanism zone and in an area known by the Turks to have US forces present".
The Pentagon has demanded that Ankara avoid actions that "could result in immediate defensive action".
Turkish Defence Minister Hulusi Akar said earlier on Saturday that the nation's military is coordinating efforts with the United States, and has denied accusations that Ankara deliberately targeted US forces in the region.
The Turkish Defence Ministry also said it had taken the necessary measures to avoid inflicting damage on a US observation post near the positions of Kurdish-led forces.
Turkey launched its military offensive dubbed 'Operation Peace Spring' on Wednesday, several days after US President Trump and Turkish President Recep Tayyip Erdogan reportedly discussed the issue in a phone call. The Trump administration quickly announced that Washington would not stand in Ankara's way although the Kurdish members of the SDF have long worked closely alongside the Pentagon to neutralize Daesh.
The offensive began after Trump ordered the removal of some 1,000 US troops from the region. Ankara stated that it had killed what it called 415 "terrorists" since Wednesday.
Turkey's move has been condemned by the international community, including by Western allies and in the Arab world. Damascus views the offensive as a violation of Syria's territorial integrity.
Turkish Foreign Minister Mevlut Cavusoglu said earlier that Operation Peace Spring was launched "in accordance with international law, Article 51 of the UN Charter and UN Security Council Resolutions on fight against terrorism".
Washington has threatened to impose economic sanctions against Ankara for the military attack.
*Daesh (ISIS, ISIL, IS, Islamic State) is a terrorist group banned in Russia and many other countries.