LONDON — Turkish forces continued their advance into northeastern Syria on Thursday as President Recep Tayyip Erdoğan said Turkey's air and ground offensive had killed more than 100 people since the operation began.

It was unclear whether all of those killed were fighting with the Kurdish-led Syrian Democratic Forces, one of the targets of the operation.

"The operation is currently continuing with the involvement of all our units," Erdogan said in a speech to members of his AK Party in Ankara, according to a translation by Reuters. He said 109 terrorists had been killed so far.

The Turkish Defense Ministry said in a tweet that it was only targeting PKK, YPG and ISIS terrorists, as well as their shelters, weapons and equipment.

Barış Pınarı Harekâtı’nın planlama ve icrasında sadece PKK/PYD-YPG ve DEAŞ’lı teröristler ile bunlara ait barınak, sığınak, mevzii, silah, araç ve gereçler hedef alınıyor. https://t.co/sw9y8Lf4Ck#MSB #TSK #BarışPınarıHarekatı🇹🇷 pic.twitter.com/zCdo7hPdWU — T.C. Millî Savunma Bakanlığı (@tcsavunma) October 10, 2019

The Syrian Observatory of Human Rights, a monitoring group based in Britain, said at least 16 members of the Syrian Democratic Forces, or SDF, had been killed.

Turkey's invasion of northeastern Syria began Wednesday, three days after President Donald Trump said he had decided to pull U.S. troops back from the area to clear the way for Turkish forces.

The Turkish defense ministry posted a video overnight of Turkish troops stalking their way through the long grass west of the Euphrates river and said the operation, codenamed Peace Spring, had been successful.

The defense ministry claimed it had struck 181 of what it called terrorist targets with the support of air forces and artillery units. For its part, the SDF claimed to have repelled a Turkish ground attack in the border area of Tal Abyad.

NBC News has been unable to independently verify claims made by Turkish forces and the militia within Syria regarding the military operations and casualties.

Trump's decision to pull back troops to allow Turkey to launch the military operation prompted a wave of criticism from the international community, as well as from Republicans and Democrats, with many accusing the Trump administration of abandoning its Kurdish allies.

The SDF is led by the Kurdish People's Protection Unity (YPG) and has been a crucial ally in the U.S. fight against the Islamic State militant group in the region. But Turkey views the YPG as an extension of the separatist Kurdistan Workers’ Party, which is considered a terrorist group by the United States.

"America is better than this. Please stand up to Turkey, Mr. President," Sen. Lindsey Graham, R-S.C., a Trump loyalist, tweeted overnight, adding that America's Kurdish allies had been "shamelessly" abandoned.

Iran said the Turkish military should immediately withdraw its troops from Syrian territory.

"Military action would not only not diminish that country’s security concerns," Iran's foreign ministry said in a statement, referring to Turkey, "but also cause financial and humanitarian damages."

The U.N. Security Council was due to meet Thursday to discuss the situation in Syria at the request of the body’s five European members: Britain, France, Germany, Belgium and Poland.

Federica Mogherini, the E.U.'s foreign affairs chief, called on Turkey to the "cease military action."

Erdogan reacted angrily to such criticism Wednesday and threatened to "open the gates" and allow millions of Syrian refugees to leave Turkey for western European nations, if the operation is framed as an invasion.

The SDF claimed overnight that a prison used to hold ISIS detainees was struck during the Turkish airstrikes. NBC News could not verify the claim. The militia has repeatedly warned that Turkey’s invasion would undermine its U.S.-backed fight against ISIS.

"As a result of our work, we were able to save the international community from the threat of the ISIS. We fought together on the front lines and spend many sleepless nights," the SDF said in a tweet. "Unfortunately our foes in the region are conspiring to destroy our people."

Meanwhile, video and stills of civilians lining up in dusty vehicles to flee the conflict zone and bombed out properties continued to circulate on social media.

Baderkhan Ahmad, a Syrian Kurdish journalist reporting from Al Qamishli, on the Syria-Turkey border, told NBC News that his hometown had been targeted overnight by Turkish mortar fire. There were clashes close to the border between Kurdish and Turkish troops, he said.

Smoke rises over the Syrian town of Ras al-Ain, as seen from the Turkish border town of Ceylanpinar, Sanliurfa province, Turkey, on Thursday. Murad Sezer / Reuters

“This morning, I haven’t heard the sound of bombardments but in the afternoon at something like 4 p.m. when it’s going to get dark, I think then they’ll start again to shell the city,” he said by phone.

Two Syrian Christians were killed in Turkish strikes overnight, according to the Syriac Military Council militia. Ahmad also reported that a Christian mother and her child were killed in his neighborhood in Qamishli.

In the streets of Ras al Ayn, one of the Syrian towns close to the Turkey border, cars raced to safety and people could be seen leaving on trucks and bringing essential belongings and blankets, The Associated Press reported.

Ahmad, who also witnessed people fleeing Ras al Ayn, told NBC news many families were fleeing farther south deeper into Syrian territory.

"We're under attack by a country who is a member of NATO," he said. "They've just left us when we're done with furthering their interests."

Saphora Smith reported from London; Aziz Akyavas reported from Akçakale, Turkey