WASHINGTON — As Turkish President Recep Tayyip Erdogan prepares for Wednesday's White House meeting with President Donald Trump, he's threatening to further inflame tensions between Washington and Ankara by purchasing Russian military fighter jets, according to multiple people familiar with the discussions.

Discussions about Turkey purchasing the fighter jets were advanced during a meeting between Erdogan and Russian President Vladimir Putin two weeks ago in Sochi, these people said. The Trump administration banned the sale of U.S.-made F-35 jets to Turkey in response to Erdogan’s purchase of a Russian missile defense system.

The possibility of Turkey, a NATO ally, acquiring more military equipment from Putin adds a new dimension to the controversy around Trump's decision to give Erdogan the prestigious platform of a White House meeting. The president is already facing push back from lawmakers who are opposed to the meeting due to the recent Turkish offensive in Syria and Erdogan's purchase of the S-400 Russian missile defense system, for which they have unsuccessfully pressured Trump to sanction Turkey.

Trump tried to appease one of those critics, Sen. Lindsey Graham, by telling the South Carolina Republican that he could sit in on Erdogan's White House meeting if he was worried about it, according to people familiar with their phone call last Wednesday.

Graham told the president that Erdogan had crossed too many lines after his attacks on America's Kurdish allies in Syria last month, according to people familiar with the discussion, while Trump made the case that Turkey had captured ISIS fighters during its invasion.

Graham told reporters Tuesday that he’ll “probably” go to the White House for the Erdogan meeting.

Senior administration officials briefing reporters Tuesday in advance of the meeting defended the president’s decision to follow through with it despite bipartisan objections and “irritants on both sides” of the alliance.

“The president is committed to direct engagement and diplomacy where it is most critical: Erdogan himself,” one official said. “[T]his is nearly a 70-year alliance. It has helped both of our countries through very, very dark times. We are not going to throw it away lightly if there is a way forward.”

A second senior administration official said the F-35s are a key part of potentially deeper economic ties between the U.S. and Turkey. “But to get there, we, as allies, need to resolve this issue of the S-400,” the official said.

Erdogan signaled last week in public remarks that he could buy the Russian fighter jets, but people familiar with the discussions said the Turkish leader may back off if Trump lifts the F-35 ban.

A spokesperson for the National Security Council declined to comment on the president's discussion with Graham or whether Trump is considering lifting the ban on F-35 sales to Turkey. White House press secretary Stephanie Grisham declined to comment.

Turkey's purchase of Russian fighter jets — or Trump's lifting of the F-35 embargo — would undoubtedly spark more backlash in Congress after months of calls for new U.S. sanctions against Turkey from both Republican and Democratic lawmakers.

On Monday a bipartisan group of members of Congress, led by House Foreign Affairs Committee Chairman Eliot Engel, D-N.Y., sent a letter to Trump calling on him to cancel his meeting with Erdogan because of the Turkish leader’s "disastrous" actions in Syria and purchase of the S-400.

"Given this situation, we believe that now is a particularly inappropriate time for President Erdogan to visit the United States," the lawmakers wrote.

Erdogan’s pursuit of additional Russian military equipment suggests he feels emboldened after Trump’s refusal to sanction Turkey over its purchase of the S-400.

"Erdogan has been following through with his threats lately," a senior U.S. official said.

After the Sochi meeting, Erdogan and Putin announced an agreement on northeastern Syria under which Russia and Turkey jointly control a large portion of what was Kurdish territory. The alliance between the two leaders has expanded Russia's influence in Syria — and the Middle East — and, coupled with Trump's decision to clear the way for Turkey's attack on the Kurds, diminished U.S. leverage.

While the Syria agreement was the main headline out of the Sochi meeting, Erdogan and Putin also advanced talks about Turkey purchasing the Russian fighter jets.

"It was point-blank discussed," one person familiar with the meeting said.

Trump announced a month ago that he had invited Erdogan to Washington for a meeting Nov. 13. But it was unclear until last week that the meeting would ultimately go forward. Turkish and U.S. officials said for days it was in flux, even though it remained on Trump's calendar.

Last Wednesday, Trump announced on Twitter that the meeting would go forward after "a very good" phone call with Erdogan. "He informed me that they have captured numerous ISIS fighters that were reported to have escaped during the conflict," Trump wrote.

The meeting was scheduled after Trump agreed during a phone call with Erdogan on Oct. 6 to pull back U.S. troops from northeast Syria to allow Turkey to attack Kurdish forces in the region who have been America's allies in the fight against the Islamic State militant group.

Trump's decision, which was followed by an order that has since been altered to withdraw all U.S. troops from Syria, set off a bipartisan firestorm and was widely criticized by his supporters and opponents.

Trump, however, has resisted sanctions against Turkey over its Syria incursion unless Erdogan in the future crosses unspecified red lines, administration officials have said.

Erdogan has said Turkey purchased the S-400 system because the U.S. wouldn't sell him the American-made Patriot system. One possible resolution could be for Trump to offer to sell Turkey the U.S.-made Patriot missile defense system and the F-35s if Erdogan does not activate the S-400 system, three people familiar with the discussions said.

Turkey's purchase of Russian military equipment is concerning for NATO allies, including the U.S., because it is incompatible with the alliance, which was formed to counter Russian aggression.

Because of overwhelming congressional support for the legislation, Trump was forced to sign the 2017 Countering America's Adversaries Through Sanctions Act, or CAATSA, under which the administration is required to adopt sanctions against Turkey for purchasing military equipment from Russia. But Trump has refused to trigger the sanctions by officially declaring that Turkey violated the law, a step the executive branch is required to take to implement sanctions.