Syria's president has assured Russia of his readiness to respect a ceasefire deal brokered by Moscow and Washington, the Kremlin said.

"A phone call took place between Russian President Vladimir Putin and President of the Syrian Arab Republic Bashar al-Assad," the Kremlin said in a statement on Wednesday.

"In particular, [Assad] confirmed the readiness of the Syrian government to facilitate the establishment of a ceasefire."

Assad described the ceasefire as an "important step towards a political resolution" for Syria's civil war, the Kremlin said in the statement on its website.

However, Putin and Assad emphasised the need to continue an "uncompromised fight" against the Islamic State of Iraq and the Levant group, the al-Nusra Front and other groups "which are included in the respective list of the United Nations Security Council", the Kremlin said.

Putin also discussed the ceasefire with the leaders of Iran and Saudi Arabia on Wednesday.

"Primary attention was focused on the Syrian issues, in particular the discussion of initiatives and proposals laid out in the Joint Statement of Russia and the United States on the cessation of hostilities in Syria," the Kremlin said in reference to Putin's call with Iran's President Hassan Rouhani.

Saudi Arabia's King Salman bin Abdulaziz al-Saud "welcomed the achieved agreements and expressed readiness for mutual work with Russia to realise them," the Kremlin said in the statement.

Saudi Arabia has been a key backer of the Syrian opposition and rebel factions, whereas Russia supports Assad, a longtime ally.

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The Syrian opposition had yet to decide whether it would commit to the US-Russian plan to stop fighting in Syria on Saturday, chief opposition negotiator Mohamad Alloush told Orient News.

Alloush said that the opposition bloc, known as the High Negotiations Committee, of which he is a member, would give the final answer.

Alloush heads the political office of Jaish al-Islam, one of the biggest rebel factions.

Also speaking on Wednesday, Turkish President Recep Tayyip Erdogan said that he was concerned that the US-Russian plan would provide an advantage for government forces and their backers while being indecisive on the terms for the Syrian opposition.

Erdogan said in a televised address that Turkey welcomed efforts towards a ceasefire in principle, but he accused the US, EU, UN, Iran and Russia of acting dishonourably in Syria by directly or indirectly permitting government forces to kill civilians.

The Syrian government said on Wednesday it was more "determined more than ever" to preserve Syria's unity after US Secretary of State John Kerry said it would be hard to hold the country together if the fighting did not stop.

Damascus was "determined today more than any time to crush terrorism and preserve the unity of Syria", according to a statement published by state media.