Vice President Joe Biden urged Syrian Kurdish forces to move back across the Euphrates River and warned them they would risk U.S. support if they don't comply.

During a visit to Ankara Wednesday, Biden delivered a stern message to Kurdish forces, telling them they "must move back across the Euphrates River," according to an Associated Press report.

If the forces refuse to do so, "they cannot – will not – under any circumstances get American support if they do not keep that commitment."

Biden is in Turkey on a mission to shore up the U.S. alliance with the key NATO ally. During his visit, the Turkish military, with U.S. support, launched a new air and ground assault on the Islamic State in Syria. The town they're targeting, Jarablus, is west of the Euphrates.

The effort is focused on clearing Islamic State militants from Jarablus and deterring Kurds from further expanding in northern Syria, according to the AP.

Appearing with Biden was Turkish Prime Minister Binali Yildirim, who said the PYD, which is the Kurdish Democratic Union Party, "shouldn't come west of the Euphrates or engage in activities in activities west of the Euphrates."

U.S.-backed Kurdish forces seized the border town of Manbij from ISIS fighters this month, forcing Turkey to retreat, the AP reported, citing Turkish state media.