The truce would mean an end to fighting between the Syrian regime and all major opposition groups except for al-Qaeda”s Syrian affiliate and the Islamic State (ISIS).
The deal would also mean an end to Russian airstrikes against rebel forces.
President Barack Obama called the Russian President Vladimir Putin about the agreement on Monday.
The White House press secretary, Josh Earnest, said: “This is a moment of opportunity, and it is the result of tenacious diplomacy on the part of Secretary Kerry. We are going to continue to try to capitalise on this moment of opportunity, and we’re hopeful that the other signatories to the agreement will do the same thing.”
In the phone call with Putin, Obama said that the US, Russia and other big powers should put pressure on the Syrian regime and armed opposition in order to alleviate the suffering of the Syirian people.
Under the agreement, all parties fighting in the civil war other than terror groups must indicate their willingness to comply with the cease-fire no later than Friday.
Both Russia and the US would develop ways to monitor the cease fire, which would include a hotline.
Putin called the agreement a “last real chance to put an end to the many years of bloodshed and violence.”
Speaking on Russian television, he said Moscow would work with the Syrian government, and expects Washington to do the same with the opposition groups that it supports.
Philip Hammond, the foreign secretary, said it would “only succeed if there is a major change of behaviour by the Syrian regime and its backers”.
He added: “Russia, in particular, must honour this agreement by ending its attacks on Syrian civilians and moderate opposition groups, and by using its influence to ensure the Syrian regime does the same.”
Syria”s government and rebels still need to accept the deal.
The Syrian War has claimed around 300,000 lives so far.