Republicans and former US officials, including some that worked in the Trump administration, have assailed President Donald Trump's decision to abandon Kurdish forces in Syria to a potential massacre at the hands of the Turkish military.

The Trump administration on Sunday abruptly announced US troops would be withdrawn from northeastern Syria, paving the way for Turkey to launch a military invasion there. Turkey views the Kurdish forces there as a threat and has been warning of launching a military operation in the region for days.

Though Republicans have largely been gentle on Trump regarding the Ukraine scandal and related impeachment inquiry, the criticism from GOP lawmakers over his Syria decision has been swift and direct.

Republican Senate Majority Leader Mitch McConnell said a "precipitous withdrawal" of US troops from Syria "would only benefit Russia, Iran, and the Assad regime."

McConnell added that it would also "increase the risk that ISIS and other terrorist groups regroup."

Republican Sen. Lindsey Graham, a top ally of Trump in Congress who sits on the Senate Foreign Relations Committee, excoriated the move.

"I hope I'm making myself clear how shortsighted and irresponsible this decision is, in my view," Graham said on Fox News on Monday morning. "This to me is just unnerving to its core."

He continued to slam Trump's decision via Twitter, saying the decision "virtually reassures the reemergence of ISIS" and "makes it difficult for the US to recruit allies against radical Islam."

Similarly, Republican Sen. Ben Sasse of Nebraska said in a statement, "If the President sticks with this retreat, he needs to know that this bad decision will likely result in the slaughter of allies who fought with us, including women and children. I hope the President will listen to his generals and reconsider."

Sasse added, "Before Turkey butchers innocent Kurds, Erdogan should carefully consider his privileged status as a NATO member. The American people don't partner with genocidal regimes."

A bipartisan group of lawmakers that just returned from a congressional delegation visit to Turkey, Afghanistan, and the Syria-Jordan border in a statement characterized the decision as a "misguided and catastrophic blow to our national security interests."

The lawmakers said the "bottom line is that these Kurdish soldiers are the first line of defense in maintaining the gains we have made against ISIS," adding that if Turkey attacks the Kurdish fighters, there's a "grave risk" that ISIS will make a comeback.

'Donald Trump is not a Commander-in-Chief'

Meanwhile, Trump's former top US envoy for the fight against ISIS, Brett McGurk, was especially critical of the president.

McGurk said in a tweet, "Donald Trump is not a Commander-in-Chief. He makes impulsive decisions with no knowledge or deliberation. He sends military personnel into harm's way with no backing. He blusters and then leaves our allies exposed when adversaries call his bluff or he confronts a hard phone call."

Nikki Haley, Trump's former US ambassador to the UN, suggested Trump was making a "big mistake" by leaving the Kurds "to die" after they played an "instrumental" role in the fight against ISIS.

"We must always have the backs of our allies, if we expect them to have our back," Haley said.

Trump was even criticized on "Fox & Friends," which tends to put a positive spin on controversies swirling around the president, with host Brian Kilmeade describing the decision as part of a "disastrous series of events."

"We defeated the caliphate; the caliphate is destroyed. We wouldn't have done that without the Kurds, who did all of our fighting," Kilmeade said. "The reason why our casualties were so low is because the Kurds did all the fighting. Now we're saying, 'OK Turks, go wipe them out or force them out.' What kind of message is that to the next ally who wants to side with us?"

'Deserting an ally in a foolish attempt to appease a foreign strongman'

Congressional Democrats also rebuked the president.

House Speaker Nancy Pelosi said Trump was "betraying" and "deserting an ally in a foolish attempt to appease a foreign strongman."

Sen. Chris Murphy of Connecticut, who sits on the Senate Foreign Relations Committee, said in a tweet that Trump had essentially invited Turkey into Syria to "wipe out the Kurds." He called Trump's decision "positively sinister."

House Intelligence Committee Chairman Adam Schiff said Trump's "impulsive behavior" has put US and allied troops in the region in danger, "and our hard won gains against ISIS at great risk."