Turkey has sent troops to eastern Syria, where fighting has raged for months between government forces and rebels.
But it’s unlikely the US will want to fight on both sides of the conflict.
Turkey’s government is already trying to control the situation by imposing a ceasefire that has largely failed to hold.
However, President Donald Trump has asked for US help to “protect innocent civilians”.
Turkey, which has a large Turkish-speaking population in Syria, is an important partner for Washington.
The US has offered to send surveillance planes, and in the past has provided logistical support for Turkey’s ground forces.
Turkey is also sending ground forces to Syria, including fighters from its own National Defence Forces.
The US wants Turkey to stop sending ground troops to Syria as well as “do not send more Turkish ground forces” to Syria.
“We need to make it clear that we have no plans to send more than 50,000 more Turkish troops to the Syrian border,” the US State Department said in a statement.
“The United States will not allow Turkey to continue to violate the international laws that it is not a party to.”
Turkey has sent about 3,500 troops to support the government in eastern Syria since 2015, the last time it was in charge of that region.
Turkey, however, is not one of the world’s most important military allies.
The United States has not been able to secure the border since 2014, and has since sought to bolster its own ground forces in the area.
Turkey has also been at odds with the US in the region.
The Turkish-backed Syrian Kurdish YPG militia is fighting on the ground in the Syrian city of Kobani against Islamic State militants, and Washington has been concerned about the Kurdish group’s influence.
The YPG, however has also faced criticism for failing to control areas it captured from the Islamic State in the first place.
Turkey also has close relations with the Kurdistan Workers Party (PKK) which has waged an armed struggle in Turkey since 1984.