The United States has given Turkey a deadline of July 31 to drop the purchase of the S-400 system, or face sanctions and removal from its F-35 fighter jet programme.

"Regardless of whatever sanctions there may be, whatever the messages from America, we've bought the S-400," Foreign Minister Mevlut Cavusoglu told reporters in Ankara.

He said Turkey was working on the date for the system's delivery, which President Recep Tayyip Erdogan has said would be in the first half of July.

"If there's an attack on Turkey tomorrow, we cannot expect NATO to protect us because NATO's capacity would only protect 30 percent of Turkey's airspace," Cavusoglu said.

Turkey will no longer allow other countries to dictate its defence purchases, he said.

Relations between Washington and Ankara have deteriorated over multiple issues, including the S-400 deal and US support for a Syrian Kurdish militia viewed as terrorists by Turkey.

Sanctions could cause damage at a time when Turkey's economy is already struggling.

Its currency lost a third of its value last year, in part due to temporary US sanctions over the detention of an American pastor.

Turkey has plans to buy 100 F-35s, and has lucrative contracts to build parts of the jet.

Erdogan said last week he would use his "good" relationship with US counterpart Donald Trump to defuse the crisis when they meet at the G20 summit in Osaka, Japan this week.