On some occasions, the monitors themselves have come under fire.
Mood said Saturday that the U.N. observers will no longer conduct patrols, staying in their locations until further notice.
This suspension will be reviewed on a daily basis, Mood said.
"Operations will resume when we see the situation fit for us to carry out our mandated activities," he said.
Meanwhile, the United States said it was tracking a Russian military cargo ship en route to Syria that is carrying weapons, ammunition and a small number of Russian troops.
U.S. intelligence believes the Russians are sending the ship to help fortify its naval base in Syria as the situation in the country continues to spiral out of control, Pentagon officials said.