U.S. Secretary of State John Kerry, speaking to reporters after a meeting in Munich that included Russia and more than a dozen other countries, said the
target for implementing the nationwide cessation of fighting was a week's time. He said all participants had agreed that Syrian peace negotiations should
resume in Geneva as soon as possible.
He said the cessation would not apply to Islamic State and other militant groups fighting in Syria.
British Foreign Secretary Philip Hammond said ending fighting could only succeed if Russia stopped air strikes supporting Syrian government forces' advance
against the opposition.
"If implemented fully and properly ... this (deal) will be an important step toward relieving the killing and suffering in Syria," Hammond said in a statement.
"A Western diplomatic source said, "We did not get a deal on the immediate end of Russian bombings, but we have a commitment to a process that if it works would change the situation."
Russian Prime Minister Dmitry Medvedev on Thursday raised the specter of an interminable conflict or even a world war if powers failed to negotiate an end to the fighting in Syria, which has killed 250,000 people, caused a refugee crisis and empowered Islamic State militants.
The first peace talks in two years between belligerents in Syria collapsed last week before they began in the face an the offensive by President Bashar al-Assad's forces, one of the biggest and most consequential of the five-year war.
Ministers at Thursday's talks wrangled over three core issues: a gradual cessation of hostilities with a firm end date, humanitarian access to cities being besieged by both sides and a commitment that Syrian parties return to Geneva for political negotiations.