The U.S. Defense Department on Friday rejected an offer by Russia’s defense minister to conduct joint air strikes against al-Qaida’s affiliate in Syria and other groups that continue to violate the shaky ceasefire.

"From where we sit today, we are not collaborating or coordinating with the Russians" on any air strikes in Syria, said Navy Capt. Jeff Davis, a Pentagon spokesman.

The only contacts between the U.S. military and Moscow involved periodic discussions to"deconflict" U.S. and Russian sorties over Syria, Davis said.

The American airstrikes in Syria were focused on the Islamic State of Iraq and Syria, or ISIS, he said.

Russian Defense Minister Sergei Shoigu proposed the U.S. join in calling on all factions in Syria's many-sided civil war to abide by the ceasefire and leave areas controlled by the Al-Nusra Front, the al-Qaida branch in Syria, by May 25.

Starting on that date, the U.S. and the Russians would then conduct strikes against Al-Nusra and any other groups that were violating the truce, Shoigu said.

"We believe the adoption of these measures will allow a transition to a peaceful process to be achieved in the entire territory of Syria," Shoigu said.

"Of course, these measures have been coordinated with the leadership of the Syrian Arab Republic," he added, meaning the regime of Syrian President Bashar al-Assad, Shoigu said, and cooperation with Assad is a non-starter for the U.S.

The ceasefire brokered by the U.S. and Russia went into effect in February, but has been repeatedly violated by Assad’s forces and opposition groups as well. The Al-Nusra Front and ISIS were excluded from the ceasefire talks.

--Richard Sisk can be reached at Richard.Sisk@military.com.