AKCAKALE, Turkey — The death toll mounted Friday as Turkish forces pushed deeper into northeastern Syria, sending civilians fleeing and prompting the U.S. to set down red lines that would trigger sanctions against Ankara.

On the Turkish side of the border, in the town of Akçakale, the streets were mostly empty Friday after mortar and rocket fire hit the town on Thursday.

Government officials told locals to stay indoors or to relocate, and overnight the population seemed to melt away. At a bazaar that was buzzing just the day before, there was no activity and those shops that were open were seemingly mostly owned by Syrian refugees.

Turkish Defense Minister Hulusi Akar said Friday that 342 people had been killed in Ankara’s operation, code-named Peace Spring, since it began two days ago.

People carry the Turkish flag-wrapped coffin of 9-month-old baby boy Mohammed Omar Saar, who was killed during a mortar and rocket assault from Syria, during a funeral ceremony in the border town of Akcakale in Sanliurfa province, Turkey, on Friday. Murad Sezer / Reuters

Those killed, he said, were members of the separatist Kurdistan Workers’ Party (PKK), considered a terrorist group by the United States, and fighters from the Kurdish People's Protection Units (YPG), that leads the Syrian Democratic Forces that has long been a crucial U.S. ally in the war on the Islamic State militant group.

The U.S. set down red lines for the offensive Thursday that would trigger economic sanctions, including ethnic cleansing and indiscriminate fire directed at civilian populations.

“If Turkey acts in a way that is disproportionate, inhumane, or otherwise goes beyond the lines the President has, in his own mind, the United States is willing to impose significant cost,” said a senior State Department official, who spoke on the condition of anonymity because they were not authorized to speak publicly on the matter.

Pro-Turkish Syrian fighters drive an armored personnel carrier across the border into Syria as they take part in an offensive against Kurdish-controlled areas in northeastern Syria launched by the Turkish military, on Friday. Nazeer Al-khatib / AFP - Getty Images

“We have not seen significant examples of that so far, but we’re very early. Turkish forces really have not engaged in great depth or in great numbers inside the border yet,” the official added.

But many of those on the ground in northeast Syria and even those as far away as Erbil, the Kurdish regional capital of Iraq, had one word to describe America's decision to step aside and allow Turkey to invade on Wednesday: betrayal.

“You can’t call Trump’s actions anything other than a betrayal,” said Lina Barakat, a representative of the Syrian Women’s Council, an organization that promotes women’s rights based in northeast Syria.

Syrian Arab and Kurdish civilians arrive in Hassakeh, Syria, after fleeing the Turkish bombardment of Syria's northeastern towns on Thursday. Delil Souleiman / AFP - Getty Images

“We fight and win against Daesh, but when we ask them to stand with us politically, they run away,” said Barakat, 40, who is not Kurdish but an Alawite, the sect of Syrian President Bashar al-Assad.

The Syrian Democratic Forces, made up of Kurdish, Arab and Assyrian fighters, broadcast reports of civilians being killed in the fighting, posting graphic pictures of bloodied bodies and maimed children.

Local journalists also published photos of bodies draped in cloth. On the other side of the border, similar photos circulated, including a child-size coffin draped in a Turkish flag.

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

The Syrian Observatory for Human Rights, a monitoring group based in Britain, said Thursday that 10 civilians had been killed in the fighting in Syria, along with 46 members of the Syrian Democratic Forces, or SDF.

Susan Archer and Aziz Akyavas reported from Akçakale, Turkey; Saphora Smith and Ziad Jaber from London; and Abigail Williams from Washington