Turkish President Tayyip Erdogan meeting US Defense Secretary Jim Mattis in 2017. Anadolu Agency via AFP

US Defense Secretary Jim Mattis reportedly urged lawmakers against halting the delivery of the advanced F-35 fighter jets to Turkey, a move that has been threatened with proposed legislation.

The first F-35 has already been transferred to Turkey but remains on US soil for a training process.

The report of Mattis’ comments to Congress came on the same day that US President Donald Trump levied criticism against Turkey for the same alleged transgression that some US lawmakers cited as a reason not to sell Turkey the jets.

Along with a concern regarding Ankara’s decision to purchase the Russian-made S-400 missile system, some members of Congress who introduced legislation to block the US sale of F-35s to Turkey mentioned the detention of US pastor Andrew Brunson.

Brunson, who is being held on charges of terrorism, was denied release in his latest hearing, prompting the US president to tweet on the matter Thursday.

“A total disgrace that Turkey will not release a respected U.S. Pastor, Andrew Brunson, from prison,” Trump tweeted. “He has been held hostage far too long.”

According to a report from Bloomberg on Thursday, Mattis wrote in a letter to Congress that “at this time, I oppose removal of Turkey from the F-35” program.

He added that such an action would lead to a “supply chain disruption” that would drive up costs for the fighter.