A ceasefire for Syria agreed by world powers is more likely to fail than succeed, Russia's top diplomat said at a meeting of foreign policy and defence officials in Munich as his country intensified airstrikes in support of President Bashar al-Assad.

Sergei Lavrov, speaking at the Munich Security Conference on Saturday, said there was a "49-per-cent" chance that a temporary truce between regime forces and the Syrian opposition would succeed.

The "pause" in fighting, agreed by the International Syria Support Group (ISSG) on Friday, is due to take effect next week to provide time for a more permanent ceasefire to be negotiated.

Lavrov's comment came as a monitoring group reported intensified Russian airstrikes against rebel-held areas near Aleppo, a critical area for Syria's internationally-recognized opposition. Moscow and Damascus mounted a joint effort to push back rebels in October.

The Munich ceasefire agreement, whose signatories include regional rival powers Saudi Arabia and Iran, will not include the Islamic State and al-Nusra Front extremist militias. Russia has insisted it is only targeting "terrorists" as part of its campaign.

US Secretary of State John Kerry called on Russia to stop targeting Syria's opposition, a condition of Friday's agreement.

"To date, the vast majority of Russia's attacks have been against legitimate opposition groups," Kerry said at the conference. "To adhere to the agreement it made, Russia's targeting must change. The entire ISSG, including Russia, will work together to ensure that happens."

Russian Prime Minister Dmitry Medvedev, who came to the Bavarian capital in place of President Vladimir Putin, said there was no evidence Russia was bombing civilians in Syria. The accusation had been earlier levelled by his French counterpart, Manuel Valls.

The Syrian Observatory for Human Rights reported dozens of fresh airstrikes as government forces advanced towards the last route into the rebel-held eastern half of Aleppo.

Another 20 airstrikes hit the town of Tel Rifaat, an enclave between Aleppo and the Turkish border, where rebels are coming under pressure from both the government and Syrian Kurdish forces.

"The intensity of the raids has increased in the past 24 hours," said Rami Abdel-Rahman, the head of the Syrian Observatory for Human Rights, adding that Al-Assad's forces are just three kilometres from Tel Rifaat.

"This intensity sets the scene for a ground advance [by Syrian troops] towards Tel Rifaat and Azaz," two rebel-held areas on the northern edge of Aleppo, he said.

The Turkish military meanwhile shelled areas near Tel Rifaat captured from rebels two days earlier by the Syrian Kurdish People's Protection Units (YPG), the Observatory and a local activist said.

Turkish Prime Minister Ahmet Davutoglu warned on Friday that his country will do "what is necessary" if it felt threatened by the YPG.

The Syrian Kurdish militia are the main US ally on the ground against Islamic State extremists, but they are also closely linked to banned Kurdistan Workers Party (PKK) rebels fighting inside Turkey.

Also on Saturday, local media reported that Saudi Arabia will deploy war planes to the Incirlik air base in southern Turkey, used by the United States and allied powers to launch attacks against Islamic State.

The exact size of any potential Saudi deployment had yet to be determined, Turkish Foreign Minister Mevlut Cavusoglu told Yeni Safak and other outlets on his plane back from Munich, adding that Turkey and Saudi Arabia could launch a ground operation.

On Friday, US Defence Secretary Ash Carter said special forces from the Gulf would help "local forces" in Syria to retake al-Raqqa, the Islamic State's de facto capital in the north. The YPG is the only US ally with significant forces on the ground near Raqqa and it controls the closest areas of the border with Turkey.

Tens of thousands of people have fled Russian airstrikes this month, creating a humanitarian crisis on the Turkish border, which remains closed to the refugees.

Al-Assad this week said he sees a risk that Turkey and Saudi Arabia, both staunch opponents of his rule, would intervene militarily in Syria.

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