The offensive can start within two days and consist of both land and air forces, according to the Turkish president.
The Kurdish YPG militia is supported by the US and during the civil war in Syria, a strong contribution was made to fighting the terrorist group IS.
But Turkey sees the militia as a branch of the Kurdish party PKK, which for 35 years fought a bloody uprising on Turkish territory.
Turkey has also previously threatened a military operation east of the Euphrates, but it has been withdrawn at the last moment after negotiations with the US.
The United States is in a difficult position when it is considered that the Kurdish forces are important in the fight against the region's Islamist terrorist groups, but at the same time are allied with the NATO country Turkey.