President Trump called Turkey's ongoing military assault in Syria a "bad idea" Wednesday as activists and war monitor reported at least seven civilians killed in the strikes.

Trump's comments come hours after Turkish President Recep Tayyip Erdogan announced the launch of Operation Peace Spring -- a mission that will "neutralize terror threats against Turkey and lead to the establishment of a safe zone, facilitating the return of Syrian refugees to their homes," escalating the long-standing feud between Ankara and Kurdish forces.

Trump was heavily criticized throughout the week following his decision Sunday to pull American troops out of northern Syria, leaving the Kurdish forces -- who have been longtime U.S. allies in the fight against ISIS in Syria -- in peril. Ankara views the Syrian Kurdish forces as terrorists allied with a Kurdish insurgency within Turkey.

"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 released by the White House. "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."

"There are no American soldiers in the area," he added.

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A spokesperson for the Kurdish-led Syrian Democratic Forces said Turkish warplanes on Wednesday have "started to carry out airstrikes on civilian areas," causing a "huge panic among people of the region."

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The Kurds requested air support from American forces in response to the strikes. But U.S. military officials tell Fox News that Trump has ordered them to not get involved.

A small group of Turkish forces first entered northeastern Syria Wednesday morning near the towns of Tal Abyad and Ras al-Ayn, Bloomberg reported. Turkey's state-run Anadolu news agency later said Turkish artillery units have been shelling suspected Syrian Kurdish targets in Tal Abayd, while two mortar shells fired from Ras al-Ayn struck the Turkish border town of Ceylanpinar.

The Britain-based Syrian Observatory for Human Rights says one fighter from the Syrian Democratic Forces has been killed so far and six others have been wounded. An SDF spokesperson also says two civilians have been killed as a result of the "Turkish bombardment" of the village of Mashrafa and four civilians were killed in Tirbespiye.

The developments come after the Kurds called on their people Wednesday to move toward the border with Turkey and “carry out acts of resistance."

"We call upon our people, of all ethnic groups, to move toward areas close to the border with Turkey to carry out acts of resistance during this sensitive historical time," read a statement from the local civilian Kurdish authority known as the Autonomous Administration of North and East Syria.

A Turkish Army tank is driven to its new position, on the Turkish side of the border between Turkey and Syria, in Sanliurfa province, on Tuesday. (AP)

In its call for “general mobilization,” the Kurds also urged the international community to live up to its responsibilities as "a humanitarian catastrophe might befall our people" in the region.

The stark message was in contrast to what was being said Wednesday north of the border.

Fahrettin Altun, the Turkish presidency's communications director, called on the international community in a Washington Post op-ed "to rally" behind Ankara, which he said would also take over the fight against Islamic State terrorists.

Turkey also is aiming to "neutralize" Syrian Kurdish militants in northeast Syria and to "liberate the local population from the yoke of the armed thugs," Altun added.

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Turkish officials who spoke to Bloomberg on condition of anonymity say the offensive will target Syrian border towns first to minimize any chance of a Kurdish state growing near its territory.

Expectations of a Turkish invasion rose after Trump on Sunday abruptly announced that American troops would step aside ahead of the Turkish push — a shift in U.S. policy that essentially abandoned the Syrian Kurds.

But Trump has also threatened to "totally destroy and obliterate" Turkey's economy if the Turkish push into Syria went too far, and cast his decision to pull back U.S. troops as fulfilling a campaign promise to withdraw from the "endless war" in the Middle East.

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The moves have been met with criticism from politicians on both sides of the aisle.

Republican critics included Sen. Martha McSally of Arizona, who said she hopes the president's plan is "reconsidered."

"I agree with President Trump's objective to not be bogged down in the Middle East, to bring our men and women home, and we really need to be looking at the rising threat of China," McSally said on "The Story with Martha MacCallum" Tuesday.

"However, we have a generational struggle and fight against Islamic terrorism,” she added.

Meanwhile, Russian Foreign Minister Sergey Lavrov accused Washington of playing "very dangerous games" with the Syrian Kurds saying that the U.S. first propped up the Syrian Kurdish "quasi-state" in northeastern Syria and is now withdrawing its support.

"Such reckless attitude to this highly sensitive subject can set fire to the entire region, and we have to avoid it at any cost," he said during a visit to Kazakhstan.

Turkish forces artillery pieces are seen on their new positions near the border with Syria in Sanliurfa province on Sunday. (AP/DHA)

Earlier on Wednesday, ISIS militants targeted a post of the Syrian Democratic Forces in the northern Syrian city of Raqqa, which was once the de facto ISIS capital at the height of the militants' power in the region.

The SDF, which is holding thousands of ISIS fighters in several detention facilities in northeastern Syria, has warned that a Turkish incursion might lead to the resurgence of the extremists. The U.S. allied Kurdish-led force captured the last ISIS area controlled by the militants in eastern Syria in March.

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In Wednesday's attack, ISIS launched three suicide bombings against Kurdish positions in Raqqa. There was no immediate word on casualties.

Also Wednesday, Iranian state television reported a surprise military drill with special operations forces near the country's border with Turkey, in Iran's Western Azerbaijan province. The TV didn't mention the expected Turkish offensive into Syria or elaborate on the reasons for the drill.

Fox News’ Jennifer Griffin, Lucas Tomlinson, Victor Garcia, Dom Calicchio and the Associated Press contributed to this report.