"Turkey's purchase of the S-400 does not amount to its withdrawal from NATO, but the move should be viewed by Ankara as a national security issue, not just a commercial decision," the officials, who spoke to a group of journalists and asked not to be named, said.
"We are still studying several options to ensure Turkey's continued participation in the North Atlantic Treaty Organization (NATO) and that bilateral relations remain undiminished or affected," one official said.
"The seriousness of the threat to F-35s for the United States and NATO allies is that the two systems cannot be deployed in the same place," he said.
Turkey, a member of the alliance, has repeatedly said it was committed to buying Russia's missile defense system despite warnings from the United States that the S-400 missiles could not be integrated into NATO's air defense system.
The State Department said last week Washington had informed Turkey that if it purchased the S-400, the United States would have to reconsider Ankara's participation in the Lockheed Martin F-35 fighter system.
Senior US officials said Washington's offer to sell Patriot missiles to Turkey was still in place and the two sides were still negotiating.
Erdogan confirmed on Thursday that Ankara could not withdraw from the agreement with Russia.
The agency believes that Turkey's insistence on buying the Russian defense system threatens to provoke a new diplomatic crisis with Washington. If Ankara goes ahead with the Russian deal, Turkey may face sanctions under a US law known as the law against US enemies through sanctions.
The latest diplomatic crisis between the two NATO members contributed to the Turkish lira falling to a record low in August. The differences between the two countries over Syria's strategy, Iran's sanctions, the detention of US consular officers and the threat of missile defense have not yet been resolved by deepening the row.