WASHINGTON – President Donald Trump lifted sanctions on Turkey Wednesday and declared a "big success" in Syria, saying Turkish officials had agreed to permanently end their military attack on Syrian Kurdish forces.

Trump's move came even as his own envoy for Syria, James Jeffrey, condemned Turkey's short but brutal military assault as deeply disruptive and said Turkish forces may have committed war crimes in its attack on the Kurds.

It also came as Russia gained a key foothold in Syria and members of Congress expressed growing concerns about Trump's decision to withdraw U.S. forces stationed at the Syria-Turkey border. Trump imposed the sanctions on Turkey last week after President Recep Tayyip Erdogan launched a military assault on the Kurds, a key U.S. ally in the fight against Islamic State terrorists.

Lawmakers in both parties pilloried Trump's decision to lift sanctions.

"It’s unthinkable that Turkey would not suffer consequences for malevolent behavior which was contrary to the interests of the United States and our friends," Sen. Mitt Romney, a Utah Republican and member of the Senate Foreign Relations Committee, tweeted after Trump's announcement.

Sen. Marco Rubio, a Florida Republican, contradicted Trump's assertion that Turkey had agreed to stop its attack on the Kurds and said Russia would be a pernicious force in the region.

"Erdogan has NOT agreed to stop all military operations in #Syria," Rubio tweeted. The senator said Russia will "remove Kurdish forces from east & west of current Turk controlled areas, including Kurdish cities" and take control of five oil fields in Syria.

While critics ridiculed Trump's claim of a victory, the president said a U.S.-brokered cease-fire between Turkey and the Kurds had held "beyond most expectations" and reiterated his decision to withdraw U.S. forces from the region. Trump said he would leave a limited number of troops in northeastern Syria to secure oil fields there.

He said his detractors want an endless, unlimited U.S. commitment in a dangerous region.

"They are the ones who got us into the Middle East mess," he said during a 15-minute speech at the White House. "Let someone else fight over this long bloodstained sand."

The president said he could reimpose sanctions if Turkey fails to honor its obligations "including the protection of religious and ethnic minorities."

The president had come under withering criticism for his decision to withdraw U.S. forces.

"Trump’s Syria decision not only will do lasting damage to America’s reputation as a trustworthy ally," said Mark Dubowitz, chief executive of the Foundation for Defense of Democracies, a hawkish Washington-based foreign policy research institute.

"It also rejects the importance of American foreign goals more than seven decades in the making," he said, to contain an expansionist Russia, support U.S. allies and orient U.S. policy away from the "fiction" that Americans will be safe from threats abroad if the U.S. retreats from the world.

In Syria, the situation on the ground remains in flux. Russia has moved to fill a power vacuum created by the U.S. departure, the Kurds fear an ethnic cleansing by Turkish forces, and an estimated 100 Islamic State fighters have escaped from Kurdish detention facilities.

On Tuesday, Russia and Turkey agreed to take joint control of a vital strip of territory along the Syria-Turkey border, a victory for Moscow as the U.S. military continued its withdrawal. Russian military police crossed the Euphrates River and entered northern Syria on Wednesday morning, according to Kremlin-controlled state media.

Meanwhile, Turkey's assault, even while suspended, has spawned a humanitarian crisis in Syria. The United Nations estimated Tuesday that about 180,000 Syrians have been forced to leave their homes or shelters, including 80,000 children, all in desperate need of humanitarian assistance.

And the pact between Russia's Putin and Turkey's leader, Erdogan, gives Moscow a crucial foothold in the Middle East. Under the agreement, Russia and Turkey agreed to work together to remove Kurdish fighters from a 20-mile zone in northern Syria.

"It is clear that the United States has been sidelined,” Sen. Bob Menendez, the top Democrat on the Senate Foreign Relations Committee, said Tuesday during a hearing on Trump's actions in Syria.

On Wednesday, Jeffrey, Trump's Syria envoy, faced a barrage of pointed questions from lawmakers in both parties on the president's decision to withdraw from Syria, which many say was a betrayal of the Kurdish fighters who helped America defeat the Islamic State's caliphate in the country.

"We all know that Trump gave Erdogan the green light to charge into northern Syria," said Rep. Eliot Engel, D-N.Y. the chairman of the House Foreign Affairs Committee. He said Trump's move had led to a "worst-case scenario" in the region.

"We handled (the Kurdish fighters) over to be slaughtered … with no warning and for no good reason," Engel said. "How could the United States do something so disgraceful."

Jeffrey defended Trump's decisions, rejecting the assertion that the president gave Erdogan a green light to attack the Kurds. He said Turkey would have attacked regardless of Trump's move, and said U.S. troops in Syria had never been given the mission to defend the Kurds against a Turkish attack.

"The Turkish incursion into northeast Syria is a tragedy" that has emboldened ISIS and created chaos in the region, Jeffrey said.

Asked Wednesday about accusations of Turkish war crimes, Jeffrey said: "We’re looking into those allegations, and we actually have a set of packages and sent a high-level demarche to Ankara demanding an explanation." That formal appeal to Turkey likely refers to the assassination of Kurdish politician Hevrin Khalaf, among other executions.

Credible reports of war crimes – being committed against Kurds by Turkish-supported militias – continue to emerge from Syria, said a U.S. official who was not authorized to speak publicly. Verifying the incidents, which have occurred despite the cease-fire, has been complicated by the withdrawal of U.S. troops from the region, the official said.

Democrats expressed particular outrage that Trump has left troops in Syria to protect the oil fields there, while saying nothing about protecting the Kurds.

"All he talks about is securing the oil," said Rep. Tom Malinowski, D-N.J., said at Wednesday's hearing. "I have not heard him say a darn thing about securing the people" living in a region now upended by Turkey's incursion.

Trump's moves in Syria have alienated even his staunchest GOP allies, including Senate Majority Leader Mitch McConnell and South Carolina Republican Lindsey Graham. And Tuesday's agreement between Russia and Turkey has only exacerbated concerns on Capitol Hill.

James Jeffrey questioning:'It's clear the US has been sidelined.' Turkey and Russia agree to joint patrols in Syria

Until two weeks ago, Kurdish forces controlled much of northeastern Syria. After an Oct. 6 phone call between Trump and Erdogan, Turkey invaded Syria and began pushing the Kurds south. Under the U.S.-brokered cease-fire, the Kurdish fighters agreed to pull back deeper into Syria, and Turkey agreed to stop its assault.

Contributing: Tom Vanden Brook and John Fritze