The strikes seemed to have been carried out near Homs, an area crucial to Assad's control of western Syria.
Adel al-Jubeir said that Assad must either leave voluntarily or face being forced to step down.
Some of the U.S.-trained rebels handed over some of their equipment to Nusra last week in return for safe passage.
"I side with the group that says 'if Russia wants to go and fight ISIS, you should let them,'" Trump said.
Western governments should be warned that including Assad will only prolong the conflict.
The president is calling for a new leader to replace Syrian President Bashar Assad to help defeat ISIS.
The two leaders are to hold face-to-face talks on the Syrian conflict on Tuesday.
A no-fly zone to stop Assad’s chemical barrel bombs? Then what?
Kerry said the Obama administration hopes talks between Russia and the U.S. will take place soon.
Russia’s recent military support of Syria is more about Ukraine than the Middle East
The army had been trained in the use of the weapons in recent months and was now deploying them.
Moscow has come under increased international pressure in recent days to explain its moves in Syria.
U.S. officials suspect Russia is reinforcing to aid Assad, Moscow's ally since the Cold War.
The White House has warned that any support for Assad is 'destabilizing and counterproductive.'
Assad knows the rebels’ backers are not interested in a decisive rebel victory.
The Syrian president says Russia is reliable, unlike "backstabbing" United States.
Russian Foreign Minister Sergey Lavrov wants the Assad government to be part of a coalition to fight ISIS.
Bashar Assad’s release of three political prisoners and a U.S.-Russia agreement offer a glimmer of hope.
The insurgents are trying to drive into the Sahl al-Ghab plain, the heartland of Assad's Alawite sect.
Smuggled out of Syria, the photos are being used to raise awareness of atrocities and push for civilian protections.
Unable to persuade more Alawites to fight for him, Assad depends upon Iranian troops to stay in power.