The U.S. backed offensive, which started on Tuesday, targets to seize the eastern Syrian town of Al-Bukamal, adding to the pressure the Islamic State faces, as it also deals with another, separate, U.S. backed operation in northern Syria aimed at driving it away from the Turkish border.
 
A rebel commander confirmed the rapid advances by the New Syria Army alliance of Arab rebel groups who had announced on Tuesday they had begun an attack to “liberate” the town along the Euphrates River in the oil rich Deir Zor province of Syria.
 
“The clashes are in the town itself but the situation has not been decided yet” told Reuters the rebel commander from Asala wa-al-Tanmiya Front, a main group within the New Syria Army, asking not to be named.
 
The “New Syria Army” formed some 18 months ago from insurgents driven from eastern Syria at the height of Islamic State's rapid expansion in 2014. Rebel sources say it has been trained with U.S. support.
 
The Syrian Observatory for Human Rights said the offensive was being mounted with backing of Western Special Forces and U.S.-led air strikes.
 
Islamic State's capture of Al-Bukamal in 2014 effectively erased the border between Syria and Iraq. Losing it would be a huge symbolic and strategic blow to the cross-border “caliphate” led by Abu Bakr al-Baghdadi. Earlier this week, in Iraq, the government this week declared victory over Islamic State in Falluja.
 
The New Syria Army's base in al-Tanf was hit twice earlier this month by Russian air strikes, even after the U.S. military used emergency channels to ask Moscow to stop after the first strike, U.S. officials say.