By Daren Butler and David Dolan

ISTANBUL (Reuters) – Turkish President Tayyip Erdogan accused the United States of supplying more weapons to Kurdish fighters in northern Syria this week, saying Washington had delivered two plane loads of arms to what Ankara considers a terrorist group.

Erdogan’s comments are likely to add to the tension between Turkey and the United States over Syria, where Washington backs the Kurdish YPG forces against Islamic State.

Turkey is part of the U.S.-led coalition against Islamic State but views the Syrian Kurdish YPG and its PYD political wing as an extension of Kurdish militants who have waged a three-decade insurgency on its own soil.

“If you think you can finish off Daesh with the YPG and PYD, you cannot, because they are terrorist groups too,” Erdogan said in comments in New York on Thursday that were broadcast on Turkish television. Daesh is an Arabic acronym for Islamic State.

“Three days ago America dropped two plane loads of weapons in Kobani for these terror groups,” he said, adding he had raised the issue on Wednesday with U.S. Vice President Joe Biden who he said had no knowledge of this.

The United States, which sees the YPG as a major strategic partner in the fight against Islamic State in Syria, air-dropped weapons to the group in the largely Kurdish town of Kobani in 2014. Erdogan said that half of those arms were seized by Islamic State fighters.

Kobani was besieged by Islamic State for four months in late 2014 and is about 35 km (20 miles) east of the Syrian border town of Jarablus, which Turkish-backed rebels seized a month ago in an operation dubbed “Euphrates Shield”.

That operation is designed to clear Islamic State fighters from Turkey’s southern border area but it has also brought Turkish and Syrian rebel forces into conflict with the YPG.

FOCUS ON ASSAD

Much of Turkey’s focus during the six-year Syria civil war has been on the need to oust Syrian President Bashar al-Assad rather than fighting Islamic State. Its recent push into northern Syria came after steady advances by the YPG.

Erdogan, who was on a visit to the United States this week, told broadcaster MSNBC that the blame for a deadly attack on a United Nations convoy rested squarely with Damascus.

“The killer responsible for that attack is Assad’s regime itself,” he said, through a translator in an interview aired on Friday.

He called again for the creation of a “safe zone” in northern Syria, an idea that has failed to gain traction with Western allies, who say it would require a significant ground force and planes to patrol.

The top U.S. general on Thursday said the military was considering arming the Syrian Kurdish fighters, and acknowledged the difficulty of balancing such a move with the relationship with Ankara.

“We are in deliberation about (what) exactly to do with the Syrian Democratic Forces right now,” General Joseph Dunford told a Senate hearing, referring to a U.S.-backed coalition that includes the YPG.

When asked whether he agreed that arming the Syrian Kurds fighters presented a military opportunity for the United States to be more effective in Syria, Dunford said: “I would agree with that. If we would reinforce the Syrian Democratic Forces’ current capabilities that will increase the prospects of our success in Raqqa.”

Raqqa is Islamic State’s stronghold in Syria.

(Additional reporting by Susan Heavey in Washington and Tuvan Gumrucku in Ankara; Editing by Ralph Boulton)