Several large explosions were reported in the Syrian town of Ras al-Ain, on the border with Turkey, as President Recep Tayyip Erdogan said that the offensive launched on October 9 had the aim to create a "safe zone" and targeted Kurdish militants and the Islamic State (IS) group in northern Syria.

Later in the day, Turkey's Defense Ministry said its ground forces had moved across the border, joined by allied Syrian opposition forces. It did not provide further details.

Turkey had long threatened an attack on the Kurdish fighters whom Ankara considers terrorists.

A monitoring group said the air strikes on the Kurdish-controlled areas killed at least 15 people.

The Syrian Observatory for Human Rights said at least eight of the victims were civilians, and two victims were killed in artillery strikes on the city of Qamishli.

The Syrian Democratic Forces (SDF) said in a statement, "We call on all the countries of the international coalition...to bear their responsibility and prevent a possible, imminent humanitarian crisis."

Earlier, Trump said in a statement that Turkey's incursion was a "bad idea" and insisted that Washington "does not endorse this attack."

Trump said Turkey had committed to "ensuring no humanitarian crisis takes place -- and we will hold them to this commitment."

Trump has come under criticism in Washington, including from senior members of his own Republican Party, over what they see as a betrayal of the Kurdish forces that fought alongside U.S. troops to defeat the IS in Syria.

The United Nations Security Council will meet behind closed-doors on October 9 to discuss the situation, diplomats said.

Turkey's offensive comes days after President Donald Trump announced he was withdrawing U.S. forces from the area.

"The Turkish armed forces and Syrian National Army [a rebel group backed by Ankara] just launched Operation Peace Spring in the north of Syria," Erdogan wrote 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 wrote.

Earlier, Turkish television reports said Turkish warplanes had bombed Syrian Kurdish positions across the border.

CNN Turk reported that several large explosions rocked Ras al-Ain, on the border across from the Turkish town of Ceylanpinar, adding that planes could he heard above.

A CNN Turk reporter said smoke could be seen rising from buildings in Ras al-Ain, one of the places from which U.S. troops withdrew earlier this week.

'Humanitarian Catastrophe'

Kurdish leaders earlier called for a "general mobilization" along the border with Turkey after Ankara said it was about to launch an offensive into northern Syria following the pullout of U.S. forces from the area.

NATO warned fellow member Turkey that its operation in northeastern Syria must be restrained.

"I count on Turkey to act with restraint and ensure that any action it may take in northern Syria is proportionate and measured," Secretary-General Jens Stoltenberg said on October 9, adding that it was important not to destabilize the region any further.

Stoltenberg told reporters in Rome that Turkey had "legitimate security concerns" and had informed NATO about its attack against Kurdish fighters in Syria.

"It is important to avoid actions that may further destabilize the region, escalate tensions, and cause more human suffering."

The outgoing head of the European Commission, Jean-Claude Juncker, also urged restraint.

"Turkey has security concerns at its border with Syria. However I call on Turkey, as well on the other actors, to act with restraint and to stop operations already...under way," Juncker told the European Parliament in Brussels.

The Kurdish-led civilian administration in northeastern Syria warned on October 9 of a "humanitarian catastrophe" in northern Syria if hostilities break out, while Russian President Vladimir Putin urged Erdogan in a telephone call to "think carefully" before launching an offensive in Syria.

"Putin called on his Turkish partners to think carefully about the situation so as not to harm overall efforts to resolve the Syrian crisis," the Kremlin said in a statement.

"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," the Kurdish-led civilian administration, known as the Autonomous Administration of North and East Syria, said.

Mustafa Bali, an SDF spokesman, tweeted: "Turkish warplanes have started to carry out airstrikes on civilian areas. There is a huge panic among people of the region."

Turkish 'Safety Zone'

Reports late on October 8 said Turkey's army had boosted its positions on the border with Syria, with dozens of military trucks, armored personnel carriers, and tanks seen heading to the town of Akcakale.

Turkish officials said that the military had struck the Syrian-Iraqi border to prevent Kurdish forces using the route to reinforce its units in the region.

The SDF said Turkish forces were shelling one of their positions in Ras al-Ain -- one of the places from which U.S. troops withdrew earlier this week, according to a British-based group monitoring the war.

Earlier, in a column published in The Washington Post , Erdogan's communications director said Turkish forces, together with the rebel Free Syrian Army (FSA), would cross the Syrian border "shortly."

Fahrettin Altun wrote that Kurdish militants in the area could either "defect" or Turkey would "have no choice but to stop them from disrupting" its fight against the IS group.

A spokesman for a small faction within the FSA was quoted as saying that 18,000 fighters were set to participate in the first stage of the offensive.

Ankara says it intends to create a 30-kilometer-deep "safety zone" along its border with Syria to resettle up to 2 million of the more than 3.6 million Syrian refugees living in Turkey.

But Juncker told EU lawmakers in Brussels that the bloc was not going to fund it. "If the Turkish plan involves the creation of a so-called safe zone, don't expect European Union to pay for any of it," he added, noting that a genuine political transition was required to resolve the Syrian conflict.

About 400,000 people have been killed in Syria's civil war, which has raged since 2011.

Turkey regards the Kurdish militias that dominate the U.S.-allied SDF as "terrorists."

Kurdish forces who helped defeat IS fighters in the war-torn country have described the U.S. pullout from northeastern Syria ordered by Trump as a "stab in the back."

Defending his move, Trump said the withdrawal affected "only 50 soldiers," while the Pentagon said the U.S. personnel were removed "to ensure their safety," but that they were not being removed from Syria.

"We may be in the process of leaving Syria, but in no way have we abandoned the Kurds, who are special people and wonderful fighters," Trump said, adding that Washington was helping Kurdish fighters "financially [and with] weapons."

Describing ties between Washington and Ankara as "very good," the U.S. president said that "any unforced or unnecessary fighting by Turkey will be devastating to their economy and to their very fragile currency."

Trump also said Erdogan will visit the White House on November 13.

Russia has provided crucial support for Syrian President Bashar al-Assad during Syria's civil war, while the United States and Turkey have backed different rebel groups.

With reporting by the BBC, AFP, Reuters, and dpa