Syrian Democratic Forces (SDF) leader Sabban Hamo said on Saturday that ISIS has been driven out of areas lying on the eastern bank of the Euphrates River.
In the official announcement on the liberation of the eastern bank of the Euphrates, the SDF, formed mostly from Kurdish units, said it has secured most of the economically valuable parts of Syria.
Territory under SDF control includes oil and gas wells and factories, water sources, three dams and large tracts of agricultural land.
If these areas were added to Afrin, the Kurdish-controlled district north of Aleppo, they would control 23 percent of Syria's 185,000 square kilometers.
Hamo said the leaders of "democratic Syria" told US officials, including presidential envoy Brett McGurk, that Washington’s influence must "stay until the political transition in Syria is achieved and a new political order is reached."
Western diplomats said US Assistant Secretary of State David Satterfield asked the opposition’s negotiating delegation in Geneva a few days ago about how to include the Kurdish SDF units in the political process.
"I have not seen any change or wavering in American support for our forces," Hamo said on US backing of SDF units.
He pointed out that there are five US military bases, two of which are found in Kobani (Ain Arab), one in al-Shadadi, one in al-Hasakah and one in al-Malikiyah.
Ankara is meanwhile seeking Russian cover for a military operation in the Afrin countryside, where the Russian army is based.
"We pose no threat to Turkey, but we will defend our territory against any attack from any party," Hamo said.
"If we are attacked, our response will be fierce and cruel."