President Donald Trump's decision to withdraw US forces from northern Syria was prompted by a Sunday phone call with President Recep Tayyip Erdogan of Turkey, The New York Times reported Monday.

Trump's decision to green light a Turkish military incursion into northern Syria, a decision that would leave the US's Kurdish allies vulnerable to attack, apparently took Pentagon officials by surprise.

Trump's impulsive norm-shattering style in phone calls with foreign leaders is coming under increasing scrutiny, with a conversation with Ukraine's leader now the focus of an impeachment inquiry.

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US President Donald Trump impulsively decided to withdraw US troops from northern Syria after a phone call with Turkish President Recep Tayyip Erdogan, according to a report Monday in The New York Times.

Details are emerging of the Sunday call, after which Trump declared plans to withdraw the troops in a move that apparently caught his own national security officials by surprise.

According to The Times, Trump "seemed to be responding instinctively" in making the decision after a phone call with Erdogan that was supposed to be focused on trade and defense assistance.

The Times reported that toward the end of the phone call Erdogan told Trump that he was moving ahead with plans to send Turkish military forces into northern Syria.

Read more: Here are the 5 major players that will feel the impact from Trump's decision to withdraw troops from Syria

The Times said Trump told his counterpart that he didn't support an incursion but did not push back further. Shortly afterward, Trump ordered the withdrawal of US special operations troops from northern Syria.

Those forces have acted as a deterrent against long-standing Turkish threats to attack Kurdish forces who are part of the Syrian Democratic Forces and have been the US's staunchest allies in the fight against the Islamic State in the country.

The SDF says it lost more than 10,000 fighters in the conflict with the Islamic State.

Trump's decision took top officials in the Pentagon and Trump's national security team by surprise, according to a report from Politico.

One National Security Council official familiar with Trump's call told Newsweek that Trump got "rolled" and outmaneuvered by Erdogan.

Turkish forces' artillery pieces seen at their new positions in Turkey's Sanliurfa province, near the border with Syria, on Sunday. Associated Press

Read more: Trump's shocking decision to pull US troops out of Syria could lead to thousands of ISIS fighters escaping from jail, Syrian official warns

"President Trump was definitely out-negotiated and only endorsed the troop withdraw to make it look like we are getting something — but we are not getting something," the National Security Council source told Newsweek.

"The US national security has entered a state of increased danger for decades to come because the president has no spine and that's the bottom line."

Trump's impulsive comments in calls with foreign leaders are under increasing scrutiny.

Read more: Trump reportedly left staffers 'genuinely horrified' in his phone calls with Putin, Saudi Arabia

Democrats have launched an impeachment inquiry into the president after details emerged of a July 25 phone call in which Trump urged Ukrainian President Volodymyr Zelensky to investigate a domestic political rival, former Vice President Joe Biden.