(Updated in second, third, fourth and final paragraphs with information on Trump's G-20 meeting with Erdoğan and wavering on sanctions)

U.S. President Donald Trump pushed back the idea of imposing hard sanctions on Turkey over its purchase of Russian missiles and advocated negotiations instead during a meeting with Republican senators on Tuesday, the Washington Post reported citing sources in the meeting.

Trump's hesitance to impose tough sanctions stems from his meeting with Erdoğan at the G-20 summit in Osaka last month, during which Erdoğan threatened to withdraw Turkey from NATO or kick the U.S. military out of a crucial airbase, NBC news quoted four sources familiar with the meeting as saying.

The threat could see the United States lose an important foothold in the Middle East region provided by the airbase in İncirlik, southern Turkey, which hosts a complement of thousands of U.S. airmen and houses American tactical nuclear weapons.

Erdoğan also sought to flatter Trump, calling him a better businessman than his predecessor, Barack Obama, and urging him to make a deal with Turkey, NBC said.

Turkey started receiving the first components from Russia of S-400 air defence missiles this month. The United States says that if Turkey deployed the missiles alongside the new F-35 stealth fighters the Turkish government also wants to buy from U.S. firm Lockheed Martin, it would allow Russia to access sensitive information on the plane’s defences and leave it vulnerable in combat.

The United States responded to the arrival of the missile parts at a base near Ankara by ejecting Turkey from the joint programme to produce parts for the F-35 jets, a move likely to hit Turkish defence companies hard.

Trump said the United States would halt the delivery of 100 F-35 jets Turkey had ordered, but did not take steps to impose sanctions under the Countering America’s Adversaries Through Sanctions Act, or CAATSA. There is bipartisan support in the U.S. Congress to take further punitive measures against Turkey. 

The president instead last week invited Republican senators to meet to specifically discuss sanctions on Turkey. But Trump seemed disinterested in the issue and wanted to talk about health care, immigration and other subjects, the Washington Post said, citing several officials familiar with the meeting on Tuesday. 

John Cornyn, one of the 40 senators who met Trump told Bloomberg that Trump was frustrated by what he saw as a lack of options in dealing with Turkey. 

James E. Risch, the U.S. Senate Foreign Relations Committee chairman who has been urging the U.S. President not to waive mandatory sanctions against Turkey, also joined the meeting. 

“Trump appeared to be advocating negotiations with Turkey instead of harsh sanctions” and openly disputed with Risch, people familiar with the closed-door meeting told the Washington Post. 

“Everyone in the room knew there were many options available and there was spirited and robust debate on all sides of the issue,” the newspaper quoted Risch spokeswoman Suzanne Wrasse as saying late Tuesday.

“What the president’s trying to do is have a much broader approach to this thing,” Senator David Perdue said after the meeting. Instead of sanctions, Perdue said, “there may be some things like free trade agreements being talked about,” according to the Washington Post. 

“Trump is wavering. That's why you haven't seen an immediate U.S. response to the first delivery” of S-400s, NBC quoted a person familiar with the discussions as saying.