Syria's opposition indicated on Wednesday it was ready for a two-week truce in Syria, saying it was a chance to test the seriousness of the other side's commitment to a US-Russian plan for a cessation of hostilities.

Combatants are required to say whether they will agree to the "cessation of hostilities" in the five-year war by noon on Friday (1000 GMT), and to halt fighting on Saturday. The United Nations hopes the planned halt will provide a breathing space for Syrian peace talks to resume.

A statement seen by Reuters from the Saudi-backed High Negotiations Committee, which groups political and armed opponents of Syrian President Bashar al-Assad, said it "views a temporary two-week truce as a chance to establish how serious the other side is in committing to the points of the agreement."

But it objected to Russia being a guarantor of the truce alongside the United States, saying Russia was a direct party to the conflict, and that the plan ignored the role Assad allies Russia and Iran were playing.

Russia intervened in the conflict on the side of Assad in September, and Iranian fighters have provided crucial support to the Syrian army in its fight against insurgents.

In Washington, US President Barack Obama expressed caution about a plan to stop the fighting in Syria, which has killed 250,000 people and created a refugee crisis in Europe.

The last round of peace talks in Geneva broke up earlier this month without progress after the Syrian government launched a Russian-backed offensive on the city of Aleppo, where more fighting was reported on Wednesday.

Obama told reporters that if some progress was made in Syria, that would lead to a political process to end the war there. "We are very cautious about raising expectations on this," he said.

Although US officials have raised the question of a political transition in Damascus, Assad, backed by Russia, shows no sign of stepping aside.

The cessation of hostilities plan does not include Islamic State or the Nusra Front, an al Qaeda affiliate that is widely deployed in opposition-held areas.

The opposition has expressed fears government forces backed by the Russian air force will continue to attack rebels under the pretext of targeting the Nusra Front.

The Syrian government, its war effort buoyed since September by the Russian air force, has accepted the cessation of hostilities agreement announced on Monday.

Assad told Russian President Vladimir Putin on Wednesday his government was ready to help implement the deal.

Putin and Assad, who held a telephone conversation, stressed the importance of a continued "uncompromising" fight against Islamic State, the Nusra Front and other militant groups.

US Secretary of State John Kerry said he had spoken to Russian Foreign Minister Sergei Lavrov and their teams would meet in the next day or so to discuss the planned ceasefire.

"I am not here to vouch that it's absolutely going to work," Kerry said in Washington. While there had to be a diplomatic solution at some point, the question was whether the time is ripe, he added.

TELEPHONE DIPLOMACY

Putin has embarked on a round of telephone diplomacy, speaking to Assad, the Saudi king, the Iranian president and the Israeli prime minister. The Kremlin described the calls as an effort to explain the substance of the US-Russia-brokered ceasefire.

The Russian Defence Ministry said it had significantly reduced the intensity of its air strikes in Syria in the past two days in areas where armed groups had expressed their readiness to join the ceasefire.

Russian state media have presented the fact that Moscow helped broker the potential ceasefire as a sign that Russia matters again on the world stage and has shrugged off what it has cast as US-led efforts to isolate it over the Ukraine crisis.

Turkish President Tayyip Erdogan said he feared the ceasefire plan would do little more than benefit Assad.

Turkey has grown increasingly frustrated by the international response to the Syrian war, in particular US support for a Kurdish militia it sees as a hostile insurgent force.

The Syrian Kurdish YPG militia told Reuters on Wednesday it would abide by the plan to halt the fighting but reserved the right to respond if attacked. The YPG is an important partner in the US-led coalition fighting Islamic State in Syria, but has also been fighting Syrian insurgent groups in northwestern Syria near Aleppo.

Ankara is also incensed by a Russian intervention that has tipped the balance of power in favour of its arch-enemy Assad.

"If this is a ceasefire that is up to the mercy of Russia, which has brutally attacked the moderate opposition and aligned with Assad under the pretext of fighting Islamic State, we fear that the fire pouring over innocent people will never stop," Erdogan said in a televised speech.

The United Nations said it was ready for a huge aid effort if the fighting stops.

The war has left 4.5 million hard-to reach people in need of humanitarian aid, the United Nations says.

The world body carried out its first airdrop of humanitarian aid to the Syrian city of Deir al-Zor on Wednesday, delivering 21 tons of relief to civilians besieged by Islamic State.

The Syrian army and Islamic State fought fierce battles on Wednesday near Aleppo, where an attack by the jihadist group has cut the main land route to the city.