There is no agreement between the United States and Russia to conduct joint air strikes in Syria, the U.S. State Department said on Friday, adding that it looked to Russia to stop truce violations by the Syrian government.

"There is no agreement to conduct joint air strikes with the Russians in Syria. What we are discussing with our Russian counterparts ... are proposals for a sustainable mechanism to better monitor and enforce the cessation of hostilities (COH)," State Department spokesman John Kirby said in a statement after Russia they stage joint air strikes on Syrian rebels, including militant Islamist group Nusra Front, who violate the truce.