ANKARA (Reuters) — The United States has offered to sell Turkey its Patriot missile defence system if Ankara promises not to operate a rival Russian system, Turkish President Tayyip Erdogan said, in what he called a significant softening in Washington’s position.

Two Turkish officials told Reuters that Turkey was evaluating the U.S. offer but that Ankara had not changed its plans for the Russian S-400 systems, which it has said it will start to activate next month.

NATO allies Turkey and the United States have been at odds over Ankara’s purchase last year of the S-400s, which Washington says are incompatible with the alliance’s defence systems.

After heavy fighting in northwestern Syria’s Idlib region this year Turkey asked Washington to deploy Patriots along its border with Syria for protection but the United States said Turkey could not have both the S-400s and the Patriots.

U.S. special representative for Syria James Jeffrey and U.S. Ambassador to Turkey David Satterfield told reporters on a conference call from Brussels that Washington was discussing with NATO what support it could offer Turkey militarily. Jeffrey also said they had considered possible responses should Russia and the Syrian government break a ceasefire in Idlib, officials said. He suggested other NATO states could individually or as an alliance provide military support to help Turkey. Nevertheless, he ruled out sending ground troops and said there still needed to be a resolution to the S-400 issue for the security relationship to move forward.

A separate Turkish official told Reuters the latest offer by Washington also included Turkey’s return to the F-35 stealth fighter jet program, which Turkey was involved in both as manufacturer of plane parts and customer for the jets. After Ankara bought the S-400s, Washington suspended its involvement in the program and threatened sanctions.