"As you know Turkey wanted to buy our Patriot system and the (former US President Barack) Obama administration wouldn't let them, and they only let them when they were ready to buy another system".
"With joy, we will come together and have talks on this issue", Erdogan said at a press conference in Ankara before boarding the plane.
The security alliance has recently been criticized by French President Emmanuel Macron, calling it "brain death, quot; due to the lack of strategic cooperation among members".
The surprise session Wednesday between the American and Turkish presidents is likely to draw attention from other world leaders who are in London this week to attend the western military alliance's annual meeting.
"The message to Turkey.is we need to move forward on these response plans and it can't be held up by their own particular concerns", Esper said as he flew to London.
Trump elaborated on his conversation with Erdoğan during another bilateral meeting in Watford with German Chancellor Angela Merkel. And that's the way I like it, ' Trump said.
Infuriating many members of Congress, Turkey shrugged off the threat of USA sanctions and began receiving its first S-400 deliveries in July.
Also on Wednesday, Trump and Stoltenberg touted a defense spending increase of a combined $130 billion by the European allies and Canada since 2016.
At the luxury hotel in Hertfordshire, northwest of central London, the leaders discussed strategy, but didn't disclose any precise steps they would take regarding China.
The leaders met in various groups in London on Tuesday before having dinner with Queen Elizabeth II in Buckingham Palace, but Macron refused to withdraw his charge that North Atlantic Treaty Organisation strategy is "brain dead" and Trump continued to insist some capitals were "delinquent" in paying their way.
Washington and Paris have long argued over NATO's goal - France opposed the 2003 Iraq war - but the new tensions will add to doubts over the alliance's future that have grown with Trump's ambivalence over US commitments to defend Europe.