Turkish President Recep Tayyip Erdogan accused the United States of sending more weapons to a Syrian Kurdish militia in defiance of Ankara's repeated insistence it is a "terrorist" organisation.
Although the US views the the People's Protection Units (YPG) militia as its most significant ground ally against Islamist militants, Ankara says the fighters are "terrorists" linked to the Kurdistan Workers' Party (PKK) which for decades has waged an insurgency in southeastern Turkey.
Erdogan said late Thursday that three days earlier the US sent "two planes with weapons" to Kobane in northern Syria for the YPG and its Democratic Union Party (PYD) political wing.
In a speech in New York after attending the UN General Assembly, Erdogan said Washington was mistaken in using the YPG as an ally in the fight against IS.
"If you think you can finish Daesh (IS) off with the PYD and YPG, you cannot, because they are terrorist groups as well," he said in remarks posted on the presidential website.
He added he had raised the issue of the alleged weapons delivery in talks with US Vice President Joe Biden but said Biden insisted he had no information.
Erdogan added the US sent arms to Kurdish militia during the battle for Kobane, a Kurdish-majority town, between IS and the YPG in 2014, saying half of the weapons fell into the hands of IS extremists.
The president's accusations risk causing further tension between the NATO allies over Washington's support for the YPG in its fight against IS.
Previously, the US has insisted that any military equipment provided to the Syrian Democratic Forces (SDF) in the past has gone only to Arab fighters.
There are about 30,000 fighters in the SDF which is made up largely of Kurds, but also has a significant Syrian Arab component.
General Joe Dunford, chairman of the Joint Chiefs of Staff, said on Thursday that the US was considering arming the SDF who would join the offensive to retake the IS stronghold of Raqa.
But Dunford said that the US would work "very closely with our Turkish allies" to assuage Ankara's concerns over the Syrian Kurds' long-term political prospects.
Turkey's presidential spokesman Ibrahim Kalin said Thursday it was "out of the question" for Ankara to join any operation to take Raqa if it included the YPG or PYD.
Turkey has over the last month sent dozens of tanks and hundreds of troops into Syria to back pro-Ankara Syrian rebels fighting IS and the YPG.