The working group of 17 countries meeting in Munich agreed "to implement a nationwide cessation of hostilities to begin in a target of one week's time," he said.
They also agreed to immediately "accelerate and expand" humanitarian aid to the war-torn country.
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"Sustained delivery will begin this week, first to the areas where it is most urgently needed... and then to all the people in need throughout the country, particularly in the besieged and hard to reach areas," Kerry said.
He also said peace talks between rebels and the Syrian government would resume in Geneva "as soon as possible".
They were derailed earlier this month as the regime began bombarding the key rebel city of Aleppo with support from Russian bombers and Iranian fighters, triggering an exodus of over 50,000 refugees.