The United States is "very very concerned" about the increased fighting in Syria ahead of more peace talks and blames the Assad military for the violence.
State Department spokesman Mark Toner said the violence contravenes the cessation of hostilities signed in February ahead of the next round of talks to start Wednesday in Geneva.
Toner said Monday that Secretary of State John Kerry wants to make sure "every extra effort is made in order to sustain and solidify the cessation of hostilities."
Syrian forces signed the temporary cease-fire with several opposition groups, but not with Islamic State or Nusra Front.
The United States and United Nations are worried that Syrian government assaults on the extremists could spread to opposition forces that are part of the cessation of hostilities.
Toner said more clarity is needed on exactly what the Syrian army has planned and who it is targeting.
The U.N. peace envoy to Syria, Staffan de Mistura, says this week's talks in Geneva are "crucially important" because they will focus on a political transition - a major issue that has held up any true progress in ending the five years of civil war.
The United States has called Syrian President Bashar al-Assad a murderer who kills his own people and says he cannot be a part of Syria's future.
Russia has said only Syrian people can make that decision.