Turkish forces launched a military offensive into Kurdish-controlled parts of Syria Wednesday, just days after President Donald Trump announced he was withdrawing U.S. troops in the region -- a move that left the Kurds, longtime U.S. allies, vulnerable to the Turkish operation.

"The Turkish Armed Forces, together with the Syrian National Army, just launched #OperationPeaceSpring" against Kurdish fighters in Syria, Recep Tayyip Erdogan, Turkey's president announced on Twitter.

"Our mission is to prevent the creation of a terror corridor across our southern border, and to bring peace to the area," he saidof Turkey's long-planned assault on the Kurds, who have been America's chief ally in Syria fighting the Islamic State terrorist group, also known as ISIS.

The move comes in defiance of international criticism and just days after President Donald Trump announced U.S. troops supporting Kurdish forces in the area would be pulled back from the border zone.

American lawmakers vowed to retaliate against Turkey and begged Trump to reconsider his decision to withdraw U.S. forces from the region.

"Pray for our Kurdish allies who have been shamelessly abandoned by the Trump Administration," Sen. Lindsey Graham, R-S.C., usually an ardent Trump supporter, tweeted amid news reports of the Turkish attack.

"Will lead effort in Congress to make Erdogan pay a heavy price," Graham said. "I urge President Trump to change course while there is still time ... "

Trump defended his decision Wednesday and called Turkey's attack a "bad idea."

"The United States does not endorse this attack and has made it clear to Turkey that this operation is a bad idea," the president said in a statement Wednesday. But, he added, "from the first day I entered the political arena, I made it clear that I did not want to fight these endless, senseless wars – especially those that don’t benefit the United States."

Trump said his administration would continue to monitor the situation closely.

"Turkey has committed to protecting civilians, protecting religious minorities, including Christians, and ensuring no humanitarian crisis takes place – and we will hold them to this commitment," the president said.

Secretary of State Mike Pompeo went further, defending Turkey's incursion into Syria.

“The Turks had a legitimate security concern,” Pompeo said in an interview Wednesday with PBS' NewsHour. “They have a terrorist threat to their south." He said the Trump administration has been trying to make sure Turkey was protected, while also trying to work with the Kurds to defeat ISIS.

Turkish warplanes began bombing parts of northeastern Syria, according to a spokesman for the Kurdish fighters in northern Syria.

Although the Kurdish fighters have been a vital U.S. ally in the fight against ISIS, Turkey sees them as terrorists.

The White House said Erdogan told Trump of his plans to move ahead with a military incursion into Syria during a phone call on Sunday night. Trump administration officials have defended the decision to remove American troops from that region, saying the president did not want U.S. forces to be in the line of fire. But critics say Trump's decision essentially gave a green light for Turkey's attack.

"The coming weeks will see a slow-moving train-wreck as US policy remains divorced from any achievable objective ... under a president that wants out altogether," Brett McGurk, who was Trump's envoy to the anti-ISIS coalition before resigning last year, posted on Twitter.

"The belief that we can now contain Turkey’s ambition into one small area is delusional," he said. "Cat’s out of bag."

Fahrettin Altun, the Turkish government's chief spokesman, said Turkey seeks to "neutralize" Syrian Kurdish fighters in northeastern Syria and to "liberate the local population from the yoke of the armed thugs." He made those comments in an op-ed in The Washington Post.

Learn more:What we know about Trump's withdrawal from northern Syria

'A reckless gamble':Four reasons critics decry Trump's 'impulsive' Syria withdrawal

Syria's Kurds have partnered with U.S.-led coalition forces in northeastern Syria fighting the Islamic State group for nearly four years. But Turkey considers some of them to be militants linked to outlawed Kurdish rebels within Turkey who have for years waged a campaign of terror aimed at securing their autonomy from Ankara.

Who are the Kurds?:A Middle Eastern people with 'no friends but the mountains'

Trump has threatened to punish Turkey economically if it does "anything outside of what we think is humane." However, both Democratic and Republican lawmakers have condemned Trump's actions, arguing that it not only poses a threat to a key U.S. ally but endangers the campaign against the Islamic State group.

Syria's Kurds have also been running detention centers in the region that are holding thousands of former Islamic State militants and their families.

In a statement late Tuesday, the General Command of the Syrian Defense Forces (SDF) – the group that's been working with U.S. troops – said the border areas of northeast Syria "are on the edge of a possible humanitarian catastrophe ... This attack will spill the blood of thousands of innocent civilians because our border areas are overcrowded."

The SDF said that at least two civilian casualties have already been caused by Turkish warplanes bombing parts of northeastern Syria. Mustafa Bali, a spokesman for the forces, said there is "a huge panic among the people of the region."

It's not clear what, if anything, Congress will do to address the looming crisis.

Rep. Liz Cheney, R-Wyo., a member of the House GOP leadership, also called for congressional action. She said Trump’s decision “paves the way for a resurgence of ISIS” and pointed to reported ISIS attacks on Tuesday in Raqqa, the group’s former stronghold in Syria.

"President Trump’s decision to withdraw U.S. forces from northern Syria is having sickening and predictable consequences. Turkey is invading Syria in reported coordination with Russian-backed forces, ISIS terrorists are launching attacks in Raqqa, and thousands of ISIS fighters are biding their time in makeshift prisons.”

Graham and Sen. Chris Van Hollen, D-Md., are working on legislation to punish Turkey for the Syrian invasion and urge Turkey's suspension from NATO. But lawmakers are on recess this week, and by the time they return Oct. 15, the military confrontation is likely to have spiraled.

Trump has so far stood by his decision.

"The United States has spent EIGHT TRILLION DOLLARS fighting and policing in the Middle East. Thousands of our Great Soldiers have died or been badly wounded. Millions of people have died on the other side. GOING INTO THE MIDDLE EAST IS THE WORST DECISION EVER MADE," he tweeted Wednesday.