The pact between the U.S. and Russia that includes Jordan and Israel, and affects southwest Syria, would be the fifth such effort to halt fighting there.
A statement says the observed "activities" recall those before an April 4 strike that killed civilians, including children. It warns the Assad government that it would "pay a heavy price."
Mouaz Moustafa has spent years advocating in Washington for the United States to get more involved in Syria. He sees President Trump's strike as long overdue.
Amnesty International says as many as 13,000 opponents of Bashar Assad have been hanged outside Damascus. NPR's Scott Simon wonders why any Syrian refugee who opposes the regime would return home.
Trump will be at the head of a nation with a relatively healthy economy and a health care system he has promised to overhaul, and he will be facing a slew of international tensions.
Homs was one of the places where the Syrian uprising began. The government has driven out its opponents in 2014, but much of the city remains in ruins, waiting to be rebuilt.
A U.K.-based monitoring group says the explosion in rebel-held Azaz, on the border with Turkey, was caused by a fuel tanker rigged with a bomb. There has been no immediate claim of responsibility.
The U.N. endorsed the "Responsibility to Protect" doctrine in 2005, calling on world powers to stop atrocities. But the secretary general says there's no longer global solidarity on the agreement.
Thousands of civilians and dozens of wounded people have been evacuated from the small section of Aleppo still controlled by surrendering rebels. But now the movement out of the city has stopped.
The Syrian leader is in his strongest position in years. He can claim control of the biggest cities, but the rebels are still a threat in the countryside and the Islamic State holds eastern Syria.