President Donald Trump and senior officials had for the past two years assumed Turkey was bluffing and would never invade northern Syria, the news site Axios reported on Sunday.

Turkish President Recep Tayyip Erdogan told Trump on October 6 that he would go ahead with a planned military operation in the region, where the US's Kurdish allies had been fighting the terrorist group ISIS.

Trump announced a US troop withdrawal from the region shortly afterward. Even Erdogan was surprised by how quickly Trump made that decision, Axios reported.

Turkey's invasion of northern Syria has plunged the region into renewed chaos, with dozens of ISIS prisoners escaping detention and the Kurds striking a deal with Syrian President Bashar Assad to prevent further Turkish incursions.

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President Donald Trump and his advisers had assumed that Turkey wouldn't follow through its threats to invade northern Syria, Axios reported on Sunday.

Trump "had been calling Turkish President Recep Tayyip Erdogan's bluff" for more than two years, the news site said, citing six sources with knowledge of the matter.

According to Axios, in previous conversations, Trump would tell Erdogan that the Turkish leader would have to "own whatever mess ensued" in the region and warned of the US presence there, and Erdogan would back down on his threats.

But having waited for US forces and their Kurdish allies — whom Turkey sees as terrorists and has long called for the destruction of — to defeat the terrorist group ISIS in the region, Erdogan sensed the US's commitment waning, Axios reported.

Erdogan has long threatened to invade northern Syria to beat Kurdish forces. Murad Sezer / Reuters

In an October 6 phone call, Erdogan appeared to challenge Trump's opposition to the plan. After Erdogan told the US president that a planned Turkish military operation would go ahead, Trump abruptly ordered the withdrawal of US forces from the region.

But according to Axios, even Erdogan was surprised by how quickly Trump acceded to his demands. He had expected Trump to push back and broker a compromise, the news site reported.

Read more: Trump's abrupt decision to pull out of Syria was reportedly made 'instinctively' at the end of his call with Turkey's president

The White House did not immediately respond to Business Insider's request for comment on the Axios report.

A Turkish-backed Syrian fighter fires during clashes in the border town of Ras al-Ain on Sunday. Nazeer Al-khatib / AFP) (Photo by NAZEER AL-KHATIB/AFP via Getty Images

The Turkish military's invasion of northern Syria has sparked a humanitarian crisis, seen dozens of "high value" prisoners loyal to ISIS escape detention, and forced the Kurds to strike a deal with Syrian President Bashar Assad's government and its Russian backers to beat back Turkey.

The Syrian government and its Russian allies are longstanding US antagonists in the Syrian war, which has spilled into its eighth year.

Trump has faced fierce criticism, including from some of his closest Republican allies, for the decision to withdraw troops from Syria. He has defended the move as part of his campaign promise to halt US involvement in what he has termed the "forever wars" in the Middle East that the US has been involved in since 9/11.

Read more: Trump's Syria retreat is a massive break from post-9/11 Republicanism

Turkish and US troops near the Turkish town of Akcakale, near the Syrian border, during joint patrols on September 8. Reuters

Details of Trump's call with Erdogan have been slowly spilling out since Trump announced the US troop withdrawal.

A National Security Council source told Newsweek earlier this month that the president had been "out-negotiated" and "rolled" by Erdogan and had failed to push back when the Turkish leader told him of his military plans.