(CNN) A handful of Republican senators on Wednesday warned Turkish President Recep Tayyip Erdogan during a candid and at times tense meeting that Turkey will face US sanctions unless it reverses course on the purchase of a Russian missile defense system, according to a senior administration official and two sources familiar with the matter.

President Donald Trump invited the five Republican senators -- all of whom have been vocal critics of Turkey's acquisition of the S-400 system -- to show Erdogan the strong opposition he faces, a senior administration official said. It was also in part an effort to show Erdogan that Trump's hands are tied on the sanctions issue.

"To the extent that Erdogan understands now the leverage and strength that the Senate has on this ... that was helpful," the administration official said.

In order to demonstrate that unified front, national security adviser Robert O'Brien held a call with the five senators before the meeting to get them on the same page about focusing on the S-400 issue, two administration officials and a Senate Republican source said.

Trump had warned Erdogan in a letter that week that sanctions were coming if Turkey deploys the S-400 system, but a source familiar with the meeting said the senators were more forthright on Wednesday.

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