ANKARA/ISTANBUL (Reuters) – Turkey has proposed to the United States that they form a working group to determine that Russian S-400 missile defense systems do not pose a threat to U.S. or NATO military equipment, Turkish Foreign Minister Mevlut Cavusoglu said on Wednesday.

The United States and Turkey have been at loggerheads over Ankara’s decision to purchase the S-400s, which are not compatible with NATO systems, from Russia. Washington has warned that proceeding with the deal could result in U.S. sanctions and the exclusion of Turkey from the F-35 fighter jet program.

“It will not be integrated into the NATO system…therefore we propose the United States to establish a technical working group to make sure that this system will not be a threat – neither to (U.S.) F-35s nor the NATO systems,” Cavusoglu told a panel in the United States.

He said it was “not our aim” to integrate the S400 system into that of the North Atlantic Treaty Organization because “it’s for our own use.” Turkey and the administration of U.S. President Donald Trump need to explain to the U.S. Congress why Ankara had to go through with the purchase from the Russians, Cavusoglu said.

Turkey says it needs the S-400s, to be delivered in July, to defend against potential security threats and President Tayyip Erdogan has repeatedly said he would not back down from the “done deal.” Cavusoglu, in Washington for a NATO summit, said he would “definitely” repeat this to U.S. counterparts during his visit.

Seeking to apply pressure, the United States halted delivery of equipment related to the stealthy F-35 fighter aircraft to Turkey, its first concrete step to block delivery of the jets due to Ankara’s planned purchase of the S-400s.

A U.S. official on Tuesday said that the purchase risked triggering sanctions. Acting U.S. Defense Secretary Patrick Shanahan later said that he expected the dispute to be resolved.

In a broader move to stop the delivery of the fighter jets, four U.S. senators introduced a bipartisan bill last week to prohibit the transfer of F-35 aircraft until Washington can certify Turkey will not take the delivery of the S-400s.

(Reporting by Tuvan Gumrukcu and Sarah Dadouch; Writing by Ali Kucukgocmen; Editing by Dominic Evans and Jonathan Spicer)