U.S. Secretary of State John Kerry is travelling to Geneva in a show of support for the cease-fire between the Syrian Government and rebel forces, Censor.NET reports citing Australian ABC.News.
The Syrian army announced a "regime of calm", or lull in fighting, late on Friday (local time), which Damascus said was designed to salvage the wider cease-fire.
But a number of rebel groups appeared to reject the "regime of calm".
"We won't accept any kind of... regional cease-fires," said a statement from a number of groups including the hardline Islamist Jaysh al-Islam, which controls areas east of Damascus.
The lull in fighting around the capital and parts of north-west 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.
Aleppo, Syria's largest city before the war, has been divided for years between rebel and Government-held zones.
Of the 250 casualties since April 22, 140 were killed in bombardments by government-aligned forces and 96 by rebel shelling.
Forty children were among the dead, according to a tally by the British-based Syrian Observatory for Human Rights.
The U.S. is working on "specific initiatives" to reduce the violence in Syria and sees stopping the bloodshed in Aleppo as a top priority, a State Department spokesman said.
In a statement detailing calls Mr Kerry has made over the past two days with UN Special Envoy Staffan de Mistura and with Riyad Hijab, a negotiator for Syrian opposition groups, State Department spokesman John Kirby said Mr Kerry had made clear the US wanted Russia to apply pressure to the Assad Government to get it to stop "indiscriminate aerial attacks" in Aleppo.