BRUSSELS (AP) — Acting U.S. Defense Secretary Mark Esper is making it clear to Turkey that it will face economic sanctions if it goes ahead with the purchase of a Russian missile defense system.

Esper met with Turkish Defense Minister Hulusi Akar on Wednesday during a NATO meeting. Officials say there was no progress in the stand-off between the two allies.

The U.S. has told Turkey that it will not be allowed to buy the F-35 fighter jet if it continues with plans to buy the Russian S-400 system.

A senior defense official traveling with Esper says that message was repeated during what was described as a frank and candid discussion. Turkey has said the S-400 purchase is a done deal.

The official spoke on condition of anonymity to discuss a private meeting.