The U.S. has enforced sanctions on Turkey over its purchase of Russian S-400 air defense systems - straining already poor relations between the two NATO allies, Reuters reported Tuesday.

The U.S. administration carried out sanctions against a Turkey military contractor and one of the agency's top executives under a 2017 accord that bans the acquisition of military hardware from blacklisted entities - like Russia.

The sanctions include a prohibition on all U.S. export licenses and loans to Turkey's military acquisition agency and a freeze on all assets of the agency's president, Ismail Demir, among other government officials.

Turkey completed its procurement of the Russian weapons system more than a year ago. 

U.S. President Donald Trump once said he was a "big fan" of Turkish President Recep Tayyip Erdogan even as the Turkish leader made decisions that put him at loggerheads with the U.S.

Turkey asked the U.S. to amend "unjust decision" and called these a "grave mistake." It said they were detrimental to mutual ties and it threatened unspecified retaliation.

The sanctions are "another manifestation of an arrogant attitude toward international law," the Interfax news agency quoted Russia's Foreign Minister Sergei Lavrov saying in Sarajevo, Bosnia and Herzegovina.

U.S. legislators have pressured Trump to impose sanctions against Turkey - but the administration had been reluctant to do so until Monday - three days after the House of Representatives approved a defense bill that required the implementation of sanctions.

After Turkey bought the Russia air-defense system, Trump booted the nation out of the U.S. F-35 program - the most advanced stealth fighter - including its involvement in helping build the plane.

Turkey's currency rose around 1% as the U.S. opted not to impose broader sanctions but analysts said the decision could still weigh on the country's economy - already in a pandemic slump and with double-digit inflation.