There are concerns about the durability of a nationwide ceasefire in Syria after government forces and their allies clashed with rebels.
The fresh fighting could put the truce – brokered by Russia and Turkey and agreed between President Bashar al Assad’s regime and opposition fighters – under pressure.
Helicopter gunships were reportedly used to attack groups, including the Fateh al Sham Front, in Wadi Barada, a rebel-held valley northwest of the capital Damascus.
The government said the al Qaeda affiliate, previously known as al Nusra Front, was excluded from the truce, which came into force at midnight on Thursday.
But the opposition said it believed the ceasefire applied to all of Syria, including areas where Fateh al Sham was present.
Islamic State and the Kurdish YPG militia have also been excluded.
It is not clear which side started the latest clashes in a country where repeated international efforts towards peace have failed.
The army began an offensive last week to retake Wadi Barada, which provides most of Damascus’ water supplies.
The military has accused rebels of deliberately targeting the infrastructure there.
Government warplanes have also carried out airstrikes in the central province of Hama, according to monitors.
Meanwhile, Russian fighter jets have hit three IS targets around the northern town of al Bab over the past 24 hours, according to Turkey’s military.
The action could be the first Russian support for Turkish army operations in the area.
The ceasefire, backed by Assad ally Moscow and opposition-supporting Ankara, has appeared to have mostly held elsewhere.
It is designed to pave the way for a political solution to the conflict after nearly six years of war, which has claimed hundreds of thousands of lives.
Russia’s President Vladimir Putin said there will soon be peace talks in Kazakhstan between Mr Assad’s government and the opposition.