The warnings were included in 90,000 leaflets the US dropped in the area over two days.
"You are within an established deconfliction zone, leave the area immediately", another leaflet read.
Army Col. Ryan Dillon, spokesman for the us -led coalition against the Islamic State group in Iraq and Syria, says the forces' continued presence is threatening usa and allied troops, which the coalition is prepared to defend.
Following Monday's advance in Iraq, a PMF spokesman said his forces are ready to march into Syria to battle the militants, but added that they would need permission from the Iraqi and Syrian governments before they could do so.
US -led coalition forces in Syria are reinforcing their garrison near the crucial Al-Tanf border crossing amid a buildup of pro-government troops nearby, a USA military official said Thursday.
Late May, the US Department of Defense said that pro-Syrian government militia that was attacked by a US-led coalition strike near al-Tanf last week has halted its advance and is working on leaving the deconfliction zone. The fighters belong to the same group that the United States attacked May 18 for moving too close to the Tanf base.
This deconfliction zone extends about 34 miles out from the city center and the Pentagon says a small pro-regime force is inside the zone while a larger pro-regime force is patrolling around the zone's perimeter.
"We are just doing everything necessary to make sure that our forces are protected", Dillion said. "We have increased our combat power in that area", Dillon told reporters. Separately, a US official confirmed that the Russians have passed on the message to the pro-regime forces.
The source noted that two offensives are taking place in the region at the same time: "A campaign initiated by the worldwide coalition led by the USA and supported by Kurdish forces in the field, and a campaign led by the Iranians to take control of the border area and ensure territorial contiguity".
Dillon said there are "a couple hundred" coalition, including U.S.
The militia claims it is in the area only to fight ISIS, Dillon said.
Additionally, hundreds of fighters from the Lebanese Hezbollah have reportedly deployed to fight USA -backed rebels in Tanf, according to reports by Fars News Agency, which is affiliated with Iran's Revolutionary Guard Corps.