The decision would cut the flow of weapons from the US to the YPG, the Kurdish militia fighting with the US-backed Syrian Democratic Forces, just six months after Trump approved the plan over Turkey's strenuous objections.
"Our discomfort regarding the provision of weapons to the YPG was conveyed to Mr. Trump once again," Turkish Foreign Minister Mevlut Cavusoglu said at a news conference Friday. "Trump very clearly said he had given instructions to not provide weapons to the YPG."
The White House did not dispute the top Turkish diplomat's account of the Friday morning call between Trump and President Recep Tayyip Erdogan, but suggested there were no impending plans to stop supplying weapons to the YPG.
Trump informed Erdogan "of pending adjustments to the military support provided to our partners on the ground in Syria, now that the battle of Raqqa is complete and we are progressing into a stabilization phase," the White House said in a statement.
Pressed about the US's specific plans, National Security Council spokesman Michael Anton told CNN the US's supply of weapons to the YPG will be "winding down over time, as the physical caliphate is destroyed."
It was not immediately clear whether Trump had overpromised in his conversation with Erdogan or if the Turkish foreign minister had oversold Trump's pledge to the US, but the lack of clarity could leave the YPG and other US-backed allies in Syria uncertain about the future of US involvement in the region.
But Cavusoglu was emphatic, saying Trump "repeated by the end of the conversation before they hung up that he ordered generals and National Security Adviser (Lt. Gen. H.R.) McMaster not to give arms."
Trump approved a plan to supply weapons to the YPG in May to help bolster the effort by US-backed forces to capture Raqqa, the self-declared capital of ISIS. The YPG is closely affiliated with the Kurdistan Workers' Party, the PKK, which has carried out terrorist attacks against Turkey, prompting concerns from the NATO ally that weapons supplied to the YPG could wind up in the hands of the PKK.
US-backed forces captured Raqqa from ISIS last month in a major milestone in the fight against the terrorist group, but the Trump administration has offered no signals it would stop arming the Kurdish militia after the city's fall.
Cavusoglu, the Turkish foreign minister, said Turkey welcomes Trump's "promise of not providing weapons to the YPG" and is waiting for the pledge to be "implemented practically."
Trump's call with Erdogan had been expected to focus on the Syrian conflict, coming days after Erdogan met with Russian President Vladimir Putin and Iranian President Hassan Rouhani to discuss Syria's future.
Trump also spoke earlier this week with Putin to discuss Syria.