Speaking in Geneva, Kerry insisted late Monday that there has been progress on a plan to restore Syria's fraying ceasefire with final details still being hammered out between world powers.

"We are hopeful, but we are not yet there yet," Kerry told reporters after meeting Saudi Foreign Minister Adel al-Jubeir and UN special envoy Staffan de Mistura.

The truce brokered by Washington and Moscow remains precarious and Kerry warned it was premature to promise success.

The top US diplomat held the Syrian military responsible for a recent attack on a Doctors Without Borders hospital in Aleppo as violence continues to escalate in fighting over Syria's second city.

Russia and Iran - which support Syria's armed forces - must now make sure that the government of Syrian President Bashar al-Assad honors the truce, Kerry said.

The US and its allies continue to back so-called "moderate rebels" and must do the same on the other side, Kerry said.

"We're trying to press this as fast as is possible," he said.

Efforts continue to restart talks

Meanwhile, the UK-based Syrian Observatory for Human Rights monitor says at least 253 civilians - including around 50 children - have been killed on both sides of divided Aleppo since April 22.

This comes despite pleas from UN Secretary-General Ban Ki-moon who appealed for the ceasefire to be extended to include Aleppo as a matter of urgency.

Syrian state television reported a Syrian army announcement on Monday that exclusion zones of fighting have been extended until 1:00 am Wednesday (2200 UTC Tuesday).

Russia and Assad's troops and allies have used the presence of al-Nusra, which was not party to a February 27 ceasefire, as an excuse to press their offensive on Aleppo.

Syria's conflict erupted in 2011 as a violent crackdown on anti-government protests. It has since escalated into a complex, multi-faceted war, with foreign fighters and a sectarian dimension; more than 270,000 people have been killed and millions displaced.

Peace talks in Geneva failed to make headway last month though UN diplomats have voiced hope they will resume next month.

jar/rc (AFP, dpa)