Syria's ceasefire has led to "reduced violence", but both sides claim the other has violated the fragile truce.
ITV News Correspondent Dan Rivers, reporting from Homs, said there have been airstrikes, but that it was impossible to know if the strikes were against so-called Islamic State and al-Nusra fighters, both of which remain "legitimate" targets under the ceasefire deal.
Dan Rivers reports from Homs on Syria's fragile ceasefire
The worry is Russia and the Syrian government are using that exemption to target other anti-government rebel groups.
"When we see a plane from the ground here we have no idea if it is targeting [IS or al-Nusra], as Russia is allowed to," Rivers said.
"Or whether it's targeting rebel groups who are supposed to be protected under this ceasefire."
The ceasefire brokered between the groups by the US and Russia began on Saturday, but explosions were reported in Damascus and in Hama soon after it came into effect.
Meanwhile, at the request of Russia and the Syrian government, the ceasefire deal does not cover IS or the al-Qaeda-linked al-Nusra Front.
Separately on Sunday, the UN and partner aid planned to take advantage of the reduced violence to deliver life-saving aid to 154,000 Syrians in besieged areas over the next five days.
Pending approval from parties to the conflict, the UN is ready to deliver aid to about 1.7 million people in hard-to-reach areas in the first quarter of 2016, the UN resident coordinator in Damascus, Yacoub El Hillo, said.