US President Donald Trump has defended his decision to abandon Kurdish fighters in Syria as fulfilling a campaign promise to withdraw from "endless war" in the Middle East,

Mr Trump declared US troops would step aside for an expected Turkish attack on the Kurds, who have fought alongside Americans for years, but he then threatened to destroy the Turks' economy if they went too far.

"If Turkey does anything that I, in my great and unmatched wisdom, consider to be off limits, I will totally destroy and obliterate the economy of Turkey (I've done before!)," he said on Twitter.

Even Mr Trump's staunchest Republican congressional allies expressed outrage at the prospect of abandoning Syrian Kurds who had fought the Islamic State group with American arms and advice.

"A catastrophic mistake," said Liz Cheney of Wyoming, the No 3 House Republican leader. "Shot in the arm to the bad guys," said Senator Lindsey Graham of South Carolina.

READ MORE: American troops start to 'abandon' Kurdish allies in Syria

Mr Trump said he understood criticism from fellow Republican leaders but disagreed with them.

Pentagon and State Department officials held out the possibility of persuading Turkey to abandon its expected invasion. US officials said they had seen no indication that Turkey had begun a military operation by late on Monday.

Mr Trump appeared largely unconcerned at the prospect of Turkish forces attacking the Kurds, who include a faction he described as "natural enemies" of the Turks.

"But I have told Turkey that if they do anything outside of what we would think is humane ... they could suffer the wrath of an extremely decimated economy," he said.

In recent weeks, the US and Turkey had reached an apparent accommodation of Turkish concerns about the presence of Kurdish fighters, seen in Turkey as a threat.

American and Turkish soldiers conducted joint patrols in a zone along the border. As part of that work, barriers designed to protect the Kurds were dismantled amid assurances that Turkey would not invade.

Mr Graham said Turkey's Nato membership should be suspended if it attacks into north-eastern Turkey, potentially annihilating Kurdish fighters who acted as a US proxy army in a five-year fight to eliminate the Islamic State's so-called caliphate.

Mr Graham, who had talked Mr Trump out of a withdrawal from Syria last December, said letting Turkey invade would be a mistake of historic proportion and would "lead to Isis reemergence".