Turkish President Recep Tayyip Erdogan reportedly told President Trump Donald John TrumpBiden on Trump's refusal to commit to peaceful transfer of power: 'What country are we in?' Romney: 'Unthinkable and unacceptable' to not commit to peaceful transition of power Two Louisville police officers shot amid Breonna Taylor grand jury protests MORE last Friday that his country would handle the remainder of Islamic State forces in Syria following a U.S. withdrawal from the country.

NBC News reports that a senior White House official says that Erdogan personally promised to Trump during a phone call on Dec. 14 that his nation's military would effectively neutralize the remaining ISIS forces operating in Syria, which is estimated to be around 30,000 militants between Syria and neighboring Iraq.

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“Erdogan said to the president, ‘In fact, as your friend, I give you my word in this,’” a U.S. official told NBC News. A second official confirmed the contents of the phone call to the news network.

The president shocked many in Washington on Wednesday when he announced a withdrawal of U.S. troops from Syria, a move that is thought to have contributed to the resignations of both Defense Secretary James Mattis James Norman MattisBiden courts veterans amid fallout from Trump military controversies Trump says he wanted to take out Syria's Assad but Mattis opposed it Gary Cohn: 'I haven't made up my mind' on vote for president in November MORE and envoy to coalition forces Brett McGurk in recent days.

According to NBC sources, that decision was made following Trump's discussion with Erdogan about Turkey's fighting capabilities in the region, while the White House has maintained publicly that Trump was merely informing the Turkish leader of his decision on the call.

On Saturday, the president wrote on Twitter that Middle Eastern countries including Turkey, a NATO ally, could effectively handle the fight against the Islamic State going forward.

“Now ISIS is largely defeated and other local countries, including Turkey, should be able to easily take care of whatever remains,” Trump tweeted. “We’re coming home!”