WASHINGTON (Reuters) - Turkey’s pending purchase of the Russian S-400 missile defense system presents a national security problem for NATO, which would not be able to deploy F-35 aircraft alongside the Russian systems, senior U.S. officials said on Thursday.

FILE PHOTO: Russian servicemen drive S-400 missile air defence systems during the Victory Day parade, marking the 73rd anniversary of the victory over Nazi Germany in World War Two, at Red Square in Moscow, Russia May 9, 2018. REUTERS/Sergei Karpukhin -/File Photo

The officials, who briefed a group of reporters on condition of anonymity, said Turkey’s purchase of the S-400 system was not tantamount to it withdrawing from NATO, but that Ankara’s purchase should be viewed as a national security issue, not a merely commercial decision.

“We are continuing to work on a range of options to ensure that Turkey’s participation in the NATO alliance and bilateral relationship can continue unabated and unimpinged,” one of the officials said.

“The gravity of the risk to the F-35 both to the United States and to NATO allies is such that the two systems cannot be co-located.”

NATO member Turkey has repeatedly said it is committed to buying the Russian missile defense system, despite warnings from the United States that the S-400s cannot be integrated into the NATO air defense system.

The U.S. State Department last week said Washington had told Turkey that if it buys the S-400 systems, the United States will have to reassess Ankara’s participation in the Lockheed Martin F-35 fighter program.

Washington has sought to persuade Turkey to instead purchase the American-made Patriot defense system, but Turkish President Tayyip Erdogan has said Ankara remains committed to the deal for the Russian S-400 surface-to-air missile defense system.

The senior U.S. officials said Washington’s offer to sell Patriots to Turkey continued and that the two sides remain in negotiations about it.

The Turkish government has already missed a “soft deadline” set by Washington to decide whether to buy a $3.5 billion Raytheon Co. Patriot missile shield system. The formal offer expires at the end of this month.

On Thursday, Erdogan repeated that it was not possible for Ankara to back out of the deal with Russia.

Turkey’s insistence on buying the Russian system risks triggering a fresh diplomatic crisis with Washington. If Ankara goes ahead with the Russian deal, Turkey also could face sanctions under a U.S. law known as Countering America’s Adversaries Through Sanctions Act.

The last diplomatic crisis between the two NATO allies contributed to driving the Turkish lira to a record low in August. Disputes over strategy in Syria, Iran sanctions and the detention of U.S. consular staff remain unresolved, and the issue of missile defense threatens to widen the rift again.