"The United States does not endorse this attack and has made it clear to Turkey that this operation is a bad idea," Trump said in a statement aimed at Turkish President Recep Tayyip Erdogan.
While talking to reporters at the White House hours later, Trump said the United States had been "speaking to both sides." At the same time he appeared resigned to the fighting.
"We've told President Erdogan how we feel," he said, "we're seeing what can be made out of a situation."
"I have gotten him to stop from virtually the first day I was in office, but they wanted to fight and that's the way it is. And they've done it for so long," Trump added.
When asked whether he was concerned that Erdogan would "wipe out the Kurds" -- who have helped the United States defeat ISIS in northeastern Syria -- Trump repeated a threat made earlier this week.
"I will wipe out his economy if that happens," the president said. "I've already done it once." Referring to the Turkish offensive, Trump said, "if he doesn't to it in as humane a way as possible," he would do something "much worse than sanctions" that some in Congress have proposed. "We will have to define that as we go along."
Mustafa Bali, a spokesman for the Syrian Democratic Forces (SDF), a U.S.-backed military alliance led by the mostly-Kurdish militia YPG, confirmed that "Turkish warplanes have started to carry out airstrikes on civilian areas" in northeast Syria.
"There is a huge panic among people of the region," Bali tweeted Wednesday.
"According to initial reports, there are casualties among civilian people," the SDF also tweeted.
Bali had earlier warned of a "humanitarian catastrophe." In the hours leading up to Turkey's invasion, he called on the U.S.-led coalition that collectively fought ISIS to "carry out their responsibilities and avoid a possible impending humanitarian disaster."
"The border areas of northeast Syria are on the edge of a possible humanitarian catastrophe," Bali said in a statement Tuesday. "This attack will spill the blood of thousands of innocent civilians because our border areas are overcrowded
Trump had for days faced harsh criticism from across the political spectrum after the White House announced Sunday night that the United States would stand aside while Turkey proceeded with an operation in northeastern Syria. Republican senators who usually stand in lock step with Trump said the move amounted to abandoning Syrian-Kurdish fighters who had fought ISIS alongside American troops.
On Monday, Trump signaled he was listening to the criticism.
"I told Turkey if they do anything that is not humane, Turkey will suffer a very decimated economy," he told reporters at the White House. "If Turkey does anything that they are not supposed to do, we will hit them so hard in their economy.”
As Turkey pushed forward with their operation Wednesday, Sen. Lindsey Graham, a Republican from South Carolina who is normally a staunch supporter of Trump, blasted Trump again.
“Pray for our Kurdish allies who have been shamelessly abandoned by the Trump Administration,” Graham tweeted Wednesday. “This move ensures the reemergence of ISIS.”
Trump said in his statement Wednesday that Turkey had “committed to protecting civilians, protecting religious minorities, including Christians, and ensuring no humanitarian crisis takes place.” The president added that the United States would “hold them to this commitment.” He did not specify how the administration would do so, although he has in recent days floated the prospect of economic punishment.
Trump also warned in a tweet Monday that, "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." He did not specify what would qualify as "off limits."
Turkey considers the YPG to be a terrorist organization and a part of the Kurdistan Workers' Party, or PKK, a Kurdish nationalist movement that is banned in Turkey. The Turkish state has been battling Kurdish separatists in its own territory for decades and has even invaded Syria to fight against Syrian Kurds several times in recent years.
"We will preserve Syria's territorial integrity and liberate local communities from terrorists," Erdogan tweeted Wednesday.
Information flowing from northern Syria was relatively limited. Turkey’s state-run news agency, Anadolu Agency, reported that Turkish howitzers targeted Kurdish fighters in the Syrian town of Tal Abyad. It also said two mortar shells fired from the Syrian side of the border had landed in the Turkish province of Sanliurfa, which borders Syria.
Smoke, meanwhile, could be seen rising near the town of Qamishli, Syria, and activists reported explosions nearby, according to the Associated Press. In the Syrian town of Ras al-Ayn – which has residential areas near the border – there was panic in the streets, the AP reported.
A U.S. official confirmed to ABC News that the Syrian Democratic Forces have halted their counter-ISIS operations. Since the U.S. forces in Syria are there to partner with the SDF in their operations, that means they, too, have effectively ceased their partnered operations. But the official stressed that the U.S. still has the ability to conduct its own unilateral counterterrorism missions inside Syria.
In his Wednesday's statement, Trump also said that Turkey was now responsible for holding thousands of ISIS prisoners the Syrian Kurds had been guarding.
"Turkey is now responsible for ensuring all ISIS fighters being held captive remain in prison and that ISIS does not reconstitute in any way, shape, or form. We expect Turkey to abide by all of its commitments, and we continue to monitor the situation closely," Trump said.
The U.S. official confirmed to ABC News that some Kurdish SDF guards have left their posts guarding prisons holding ISIS prisoners. “There has been some reduction in manning” at those posts that began yesterday," said the official. The reduction in forces is “certainly” a concern said the official.
The official acknowledged that there are some SDF prison facilities located in the zone that Turkey is targeting.
ISIS, which grew out of al-Qaeda in Iraq, took root in northern and eastern Syria in 2013 during the country's ongoing civil war, after seizing swaths of territory in neighboring Iraq. The jihadist group sought to overthrow Syrian President Bashal al-Assad's regime and establish a caliphate.
In 2014, as ISIS carved out territory in Syria, the United States led a coalition of countries to conduct airstrikes targeting the group. The military coalition ultimately teamed up with the Syrian Democratic Forces to liberate the ISIS-held territories in northeastern Syria -- a partnership that angered Turkey.
The U.S.-led coalition provided crucial air support, weapons and logistical advice, but it was the Kurdish-led Syrian Democratic Forces that sacrificed many of its soldiers on the battlefields. ISIS lost control of its last foothold in Syria earlier this year.
Negotiators from the United States and Turkey reached an agreement on Aug. 7 to create a safe zone in northeastern Syria in order to secure the territory, which is now under Kurdish control, and keep out ISIS. But Erdogan still threatened to take up an offensive against the Kurds.
"The Kurds fought with us, but were paid massive amounts of money and equipment to do so. They have been fighting Turkey for decades," Trump tweeted on Monday morning. "I held off this fight for almost 3 years, but it is time for us to get out of these ridiculous Endless Wars, many of them tribal, and bring our soldiers home."
ABC News' Luis Martinez contributed to this report.