According to NBC News, Trump demanded the commitment from Turkish President Recep Tayyip Erdogan as one of several conditions which need to be met before US forces mostly exit the region. 

"There are objectives that we want to accomplish that condition the withdrawal," said Bolton. 

He spoke to reporters traveling with him to Israel and Turkey as he tried to clarify Trump's Syria withdrawal policy for allies. He's meeting with Israeli officials Sunday and Monday, and with Turkish officials, including Erdogan, on Tuesday.

Since Trump abruptly announced on Dec. 19 that all U.S. forces in Syria would exit immediately, administration officials have shifted the timing to say it would happen more slowly. Officials are now setting a series of conditions for withdrawal that must first be met, which Bolton described as "policy decisions that we need to implement." -NBC News