"The issue of S-400 is an issue directly related to our sovereignty and we will not backtrack from that," Erdogan said in a televised speech.

"God willing, the delivery of the S-400 will start next month," he said.

"In order to meet its security needs, Turkey... does not need to get permission, let alone bow to pressure."

Erdogan has vowed to use his good relations with US counterpart Donald Trump to defuse tensions when they meet on the sidelines of the G20 summit in Osaka, Japan later this week.

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

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

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