Nearly 30 air strikes hit rebel-held areas of Aleppo on Saturday as a temporary "calm" declared by Syria's military took effect around Damascus and in the northwest.
It was the ninth day of deadly bombardments in Aleppo, which has borne the brunt of increased fighting that has all but destroyed a February ceasefire and killed nearly 250 people in the northern city since April 22, a monitoring group said.
It also contributed to the break up of peace talks in Geneva, which the main opposition walked out of last week.
The Syrian army announced a "regime of calm," or lull in fighting, late on Friday, which Damascus said was designed to salvage the wider ceasefire.
A number of rebel groups appeared to reject the "regime of calm," however.
"We won't accept any kind of... regional ceasefires," a statement from a number of groups including Jaysh al-Islam, which controls areas east of Damascus, said.
It said the main armed opposition as a whole reserved the right to respond to attacks on rebel factions in any part of the country, and criticized the United States for not doing enough to stop government bombardments.
The lull in fighting around the capital and parts of northwest coastal province Latakia, announced by the army, appeared to hold through most of Saturday but the bombing continued in Aleppo which was excluded from the plan.
Anas Al Abde, president of the Turkey-based opposition Syrian National Coalition, accused the government of violating the February truce "daily." The opposition was ready to reinstate the wider truce, but reserved the right to respond with force to attacks, he said.
All sides have accused each other of truce violations.
The United States said it was working on "specific initiatives" to reduce the violence in Syria and sees stopping the bloodshed in Aleppo as a top priority, a U.S. State Department spokesman said on Saturday.
In a statement detailing calls U.S. Secretary of State John Kerry has made over the past two days with UN Special Envoy for Syria Staffan de Mistura and with Riad Hijab, chief coordinator of the main opposition HNC bloc, State Department spokesman John Kirby said Kerry had made clear the United States wanted Russia to apply pressure to the Assad government to get it to stop "indiscriminate aerial attacks" in Aleppo.
Kerry is travelling to Geneva on May 1-2 to discuss the Syrian conflict with his Jordanian and Saudi counterparts as well as de Mistura, the State Department said on Saturday.