(Recasts, adds comments from Turkish official and Pentagon official, adds background)
By Humeyra Pamuk
WASHINGTON, May 13 (Reuters) - The United States has asked Turkey to delay taking delivery of the S-400 Russian missile defense system, currently scheduled for July, in return for potentially approving the formation of a working group that Ankara has sought to establish, a source familiar with the matter said on Monday.
But a senior Turkish official reiterated Ankara's position that it would not back down from its planned purchase of the system, a procurement that has widened a diplomatic rift between the two NATO allies.
"We will buy the s-400s in July. Our position has not changed," he said.
But the source said talks on the issue continued.
U.S. officials have called Turkey's planned purchase of the S-400 missile defense system "deeply problematic," saying it would risk Ankara's partnership in the joint strike fighter F-35 program because it would compromise the jets, made by Lockheed Martin Corp.
The United States and other NATO allies that own F-35s fear the radar on the system will learn how to spot and track the jet, making it less able to evade Russian weapons.
Ankara has been pushing Washington to establish a working group to assess the risks the system would be posing to the F-35 jet. U.S. Secretary of State Mike Pompeo had dismissed the proposal but there has been renewed effort in Washington to try to use the offer to open a channel for further dialogue with Ankara, another source briefed on the matter said.
The disagreement is the latest in a series of diplomatic disputes between the United States and Turkey. They include Turkish demands that Washington extradite cleric Fethullah Gulen, differences over Middle East policy and the war in Syria, and sanctions on Iran.
In protest at Ankara's planned Russian missile defense system purchase, the United States in late March halted delivery of equipment related to the stealthy F-35 fighter aircraft to Turkey.
On Monday, the top civilian in the U.S. Air Force, Secretary Heather Wilson, confirmed to reporters in Washington on Monday that shipments of F-35 support gear to Turkey were still halted. She added that discussions were ongoing regarding those shipments.
But Wilson said Turkish pilots continued their F-35 training program in Arizona.
Reporting by Humeyra Pamuk; Additional reporting by Orhan Coskun in Ankara and Mike Stone in Washington; Editing by Peter Cooney