Syrian government forces and rebels clashed on May 6 shortly after a Russian-led deal to establish a cease-fire within Syrian "safe zones" took effect, monitors said.
Syrian fighter jets fired at the rebel-held village of Al-Zalakiyat in the northwestern province of Hama, and government forces shelled the nearby towns of Kafr Zita and Latamneh, the Syrian Observatory for Human Rights said.
The observatory said a loud explosion also rocked the Teir Malla area, north of Homs, after the truce was to take effect at midnight on May 5.
Activists said government forces fired a rocket into the rebel-held area.
The renewed fighting occurred as U.S. Secretary of State Rex Tillerson spoke by phone with Russian Foreign Minister Sergei Lavrov about efforts to reduce the violence in Syria and move to a political solution to the six-year civil war, the State Department said.
Several previously negotiated cease-fires in Syria have fallen apart.
The latest truce is supposed to occur in designated "safe zones" within Syria and was announced this week by Russia, Iran, and Turkey. It gained support from the Syrian government, but rebel groups opposed the safe zone plan, calling it an effort to "partition" the country.
Based on reporting by dpa and Reuters