A landmark truce is to take effect in Syria on Saturday, the US and Russia announced, but the “cessation of hostilities” does not include the Islamic State and Al-Nusra Front, the main jihadist factions.

The leading opposition group in the five-year conflict gave its conditional acceptance to today’s announcement, but Israel said it was sceptical the deal would hold, and analysts warned any pause in the fighting would be dependent on Russia, Iran and President Bashar al-Assad.

The announcement came a day after the deadliest jihadist attack in Syria’s brutal civil war, with 134 people — mostly civilians — killed in a series of blasts near Damascus.

In a joint statement, Washington and Moscow said the partial truce would begin at midnight Damascus time, suspending a vicious conflict that has left more than 260,000 people dead and seen half the population displaced.

“If implemented and adhered to, this cessation will not only lead to a decline in violence, but also continue to expand the delivery of urgently needed humanitarian supplies to besieged areas and support a political transition to a government that is responsive to the desires of the Syrian people,” US Secretary of State John Kerry said.

US President Barack Obama and Russia’s President Vladimir Putin discussed the deal by phone, the White House said.

“This is a moment of opportunity and we are hopeful that all the parties will capitalise on it,” White House press secretary Josh Earnest said.

Putin said Moscow would do “whatever is necessary” to ensure Damascus respects the agreement.

“We are counting on the US to do the same with its allies and the groups that it supports,” he said.

The two global powers are pursuing separate air wars in Syria, with Russia pounding rebel targets and a US-led coalition focused on IS jihadists.

UN Secretary General Ban Ki-moon called the deal a “long-awaited signal of hope”, and urged all sides to abide by it.

There was no immediate reaction from Damascus, but the main grouping of opposition factions said it “agreed to respond positively to international efforts to reach a truce deal”.

Israeli Defence Minister Moshe Yaalon said he was sceptical that the ceasefire would succeed.

“It is difficult for me to see a ceasefire while Daesh (the Islamic State group) and the Al-Nusra Front (the Syrian Al-Qaeda affiliate) are not part of the process and the Russians say they will strike both organisations,” he said in a statement.

The rise of IS, which has seized large parts of Syria and neighbouring Iraq and become the pre-eminent global jihadist group, has focused attention on the need for a solution.

Analysts also had reservations about the deal.