A major hospital along the border has been abandoned and tens of thousands are fleeing south as Turkish forces continue to launch a military offensive into Kurdish-controlled parts of Syria.

Secretary of State Mike Pompeo warned Turkish leaders against the offensive on Friday, threatening a "real cost" to Turkey if it continues down its current path.

"We’re using every economic and diplomatic tool to convince them to cease this activity," Pompeo said in an interview with The Tennessean. "It’s a bad thing, and we’re working to convince [Erdogan] that moving into Syria this way is putting at risk the lives of the Kurdish people, and others as well, Christians and other ethnic minorities and religious minorities."

U.S. officials say an American outpost in northeastern Syria has been vacated after coming under fire from Turkish artillery, the Associated Press reported. Turkey says it didn’t target the U.S. outpost but was responding to fire from Kurdish groups nearby.

Towns and villages along the border have been hit with heavy shelling over the past few days, and residents are fleeing south to seek shelter. Doctors Without Borders, the international medical charity also known as MSF, said one of its hospitals along the border has closed.

The humanitarian impact of Turkey's "Peace Spring" offensive is already being felt. The United Nations said Friday that an estimated 100,000 people have already been displaced and are seeking shelter in host communities, collective shelters and schools.

“This escalation can only exacerbate the trauma that the people of Syria have already endured through years of war and of living in precarious conditions,” said Robert Onus, MSF emergency manager for Syria, in the release.

More from Syria:Turkish strikes hit civilians as Syria offensive intensifies

On the Syrian side, at least seven civilians have been killed, according to the Kurdish Hawar news agency and the Syrian Observatory.

Turkish officials say Kurdish militia have killed at least nine civilians, including a 9-month-old boy and three girls under 15, by firing mortars into border towns, the Associated Press reports.

Turkey's Defense Ministry said four soldiers have been killed in the offensive, according to the Associated Press.

There are various reports of how many Kurdish fighters have been killed.

The military offensive is also raising concerns about the fate of Kurdish-run prison camps holding thousands of Islamic State prisoners.

Kurdish forces on Friday posted a video to Twitter saying that relatives of Islamic State families were "rioting" and attempting to escape."

Turkey's military stepped up airstrikes and a ground offensive into Kurdish-held parts of northeastern Syria after President Donald Trump's decision to withdraw U.S. troops who fought alongside Kurds battling the remnants of the Islamic State group.

Trump said Monday that he was delivering on a campaign promise to pull U.S. troops out of "these ridiculous endless wars."

Turkey launched the assault because it views a Kurdish militia that dominates the U.S.-backed Syrian Democratic Forces, or SDF, as a terrorist group. This group, the People's Protection Unit is aligned with the Kurdistan Workers' Party, or PKK, which has fought for autonomy in eastern Turkey for years. As part of that effort, the PKK carried out a wave of bombings and other attacks in Turkey.