(CNN) Secretary of Defense James Mattis criticized Turkey for its "authoritarian drift" and its record on human rights but still wants the NATO ally to receive the F-35 Joint Strike Fighter, according to a letter to a senior lawmaker on Capitol Hill.

"I understand and agree with Congressional concerns about the authoritarian drift in Turkey and its impact on human rights and rule of law including the detainment of American citizens such as Pastor Brunson," Mattis said in a letter to the chairman of the House Armed Services Committee, Republican Rep. Mac Thornberry of Texas, dated earlier this month and obtained by CNN.

"The Administration is pressing Turkey on these issues as well as the potential acquisition of the S-400 air defense system," Mattis added, referring to the Russian-made weapons system that has sparked concerns on Capitol Hill and in the Pentagon due to fears that its use by Turkey could enable Moscow to gather sensitive intelligence on NATO and the US military.

"The F-35 is the most advanced aircraft, the most advanced NATO aircraft, and if Turkey goes forward with the acquisition of the S-400, it will allow the Russians to collect information on how to best attack an F-35 fighter," Sen. Chris Van Hollen, a Maryland Democrat who has led efforts to block Ankara from receiving the jet, told CNN last month.

Yet despite these criticisms of Turkey, Mattis said he opposed recent congressional efforts to insert language into the defense budget, the National Defense Authorization Act, aimed at blocking Turkey from receiving the F-35 stealth jet

Read More