BEIRUT — Violence significantly decreased in much of northern and western Syria on Saturday, the first day of a truce brokered by the United States and Russia, even amid reports of scattered violations, including incidents of bombing, shelling, and at least one aerial attack.

Russia said it was suspending, at least for the day, the airstrikes it had been conducting for five months in support of the Syrian government, and residents in many areas enjoyed a few tentative hours of calm.

“There’s a sense of relief,’’ said Hassan Taqieddine, an anti­government activist in Douma, a besieged suburb of the capital, Damascus, adding that while there had been no shelling so far, most were staying indoors.

The relative lull came after several particularly bloody days, including dozens of airstrikes and barrel bombs dropped from government helicopters in the Damascus suburbs and in the northern province of Idlib.

The United States and Russia have invested considerable political capital in the truce, although it has not been formally signed by either the Syrian government or its opponents. It is the first such attempt since a UN-brokered truce in 2012 that broke down within hours.

The deal still faces major obstacles, not least a lack of clarity on who is going to stop shooting at whom and how the truce will be monitored. Russia and the United States have set up parallel monitoring centers, at Russia’s air base in Syria and in Amman, Jordan.

The truce does not include the powerful Nusra Front, which is affiliated with Al Qaeda and fights alongside armed opposition groups. The deal also does not cover the self-declared Islamic State. Neighboring Turkey has said the truce does not cover Syrian Kurdish militias it considers to be terrorist organizations. And all parties say they have the right to retaliate in self-defense.

Insurgents fighting President Bashar Assad, including some supported by the United States and its allies, have said they expect the Syrian government and its Russian allies to continue targeting them under the pretense of hitting Al Qaeda or the Islamic State.

Such worries were amplified when Russia issued a map of areas it expected to fall under the partial cease-fire — tiny patches amid larger areas it considered controlled by terrorists.

But Saturday, such attacks on other insurgents did not seem to be taking place on a wide scale. In Moscow, the Defense Ministry said the warplanes it had deployed to Syria were not conducting attacks Saturday.