It comes as the United Nations says it has completed its first aid airdrop to 200,000 civilians in the country.
A little more than a day before a provisional ceasefire is due to come into effect in Syria, doubts remain over its implementation.
The main opposition group, the High Negotiations Committee, has until midday on Friday 26 February to say whether it will agree to the truce.
The truce, brokered between the United States and Russia, begins from midnight on Friday, Syrian time.
But a spokesman for the opposition group, Salem Al-Meslet, says questions remain over Russia's commitment.
"We hope that you know (they) are serious about it, because we have real doubts, and we do not ... to be frank with you, we do not trust Russia, we do not trust this regime. We are sure that Russia will do something, but the problem in this agreement, if they target al-Nusra -- we are not defending al-Nusra, we are against al-Qaeda, we are against ISIS, Daesh -- but maybe they will use that as an excuse to target moderate fighters, to target the Syrian Free Army. There has to be some assurance, you know, from the states, especially from the United States."
The cessation in hostilities does not cover groups the United Nations considers terrorists, such as the self-proclaimed Islamic State, or IS, and the al-Qaeda-linked al-Nusra Front.
But speaking after a meeting with Jordan's King Abdullah, US president Barack Obama says the break in fighting could lay the groundwork for future peace negotiations.
"If, over the next several weeks, we can see some lessening of the violence that's been wracking that country, then that provides us a basis to build a longer-term ceasefire, both in the north and the south, and allows us to move forward on the political transition that ultimately's going to be necessary to bring an end to the civil war in Syria."
Their remarks come after Russian foreign minister Sergei Lavrov accused the United States and its allies of trying to undermine the deal by suggesting Russia may not keep its word.
Meanwhile, Turkish president Recep Tayyip Erdogan has voiced concern the plan would actually benefit Syrian president Bashar al-Assad.
"We endorse a ceasefire that would relieve our Syrian brothers, but it is worrying to see that, with this ceasefire, Assad's regime that is responsible for the killing of more than half a million people, and the powers who are backing him, receive open support, while a hesitant tone is used towards the opposition."
The United Nations says it is standing by to begin a massive humanitarian-aid operation should the fighting in Syria stop.
More than a quarter of a million people have been killed in the violence.
The United Nations has recently completed its first airdrop of aid, 21 tonnes to civilians in the city of Deir al-Zor.
An official from the UN Humanitarian and Emergency Relief department, Stephen O'Brien, says the Syrian government is ignoring its human-rights responsibilities.
"While the United Nations and its partners are ready to take advantage of any opportunity to reach people in need, granting access should never be dependent on political negotiations or ad hoc deals on the ground. Protecting civilians and facilitating humanitarian assistance are legal obligations that are incumbent on all parties to the conflict at all times and for all types of assistance. It is a fundamental and irrefutable tenet of international humanitarian law, and it must be respected. In that regard, I once again call upon the government of Syria to urgently approve the over 40 outstanding requests for inter-agency convoys to deliver assistance to hard-to-reach and besieged areas."
The impending ceasefire appears to have done little in the city of Aleppo, where Syrian army forces and IS fighters continue to battle.
A recent attack destroyed the main land route to the city, and there are reports the nearby town of Khanaser has fallen to IS.
