By Friederike Heine and Ramadan Al-Fatash

Munich/Cairo (dpa) - Hopes of a successful truce and the speedy delivery of humanitarian aid to besieged areas of Syria dwindled on Sunday as regime forces - backed by Russian airstrikes - pushed ahead with an offensive against rebels in the northern province of Aleppo.

US Senator John McCain, Riyad Hijab, chief negotiator for the Syrian opposition, and deputy UN chief Jan Eliasson were among diplomats and foreign policy experts to cast serious doubt on an agreement by world powers to implement a truce and deliver aid by next week.

"My scepticism rests simply on the nature of our adversaries' ambitions," McCain said at the Munich Security Conference, adding that President Vladimir Putin's Syrian intervention was an attempt to re-establish Russia as a major power by exacerbating the refugee crisis in order to divide NATO and undermine the European project.

"If Russia and the [Syrian President Bashar al-]Assad regime violate this agreement, what are the consequences? I don't see any. Common sense will not end the conflict in Syria - it takes leverage," he said.

Hijab accused Moscow and Damascus of pursuing a "strategy of forced displacement," adding that neither party was invested in a truce.

The United Nations was still waiting for security guarantees that would allow the delivery of humanitarian aid to besieged areas of Syria that world powers had scheduled for Monday.

The process is more complicated than expected, Eliasson told dpa on the sidelines of the conference.

"We need quick action from their side," he said, adding that aid convoys were ready to go but it was unclear when security assurances from Damascus would come.

Key players in the conflict were still trying to prop up hopes for a truce, with the Kremlin putting out a statement that a phone conversation between Putin and US President Barack Obama in the wake of the agreement had been "frank and constructive."

According to the Kremlin, Putin reiterated the importance of coordinating US and Russian military efforts in the fight against the Islamic State and al-Nusra Front militias, two extremist groups which are exempted from the truce. Russia has insisted its air campaign only targets "terrorists."

The truce "requires opposition groups to stop fighting but it allows Russia to continue bombing 'terrorists,' which it insists is everyone, even civilians," McCain charged.

Al-Assad's forces, backed by Russian airstrikes, pressed ahead Sunday with a major attack against the rebel-held towns of Anadan and Hreitan on the northern outskirts of Aleppo, according to activists.

Government forces have made significant territorial gains in the area since they started their onslaught earlier this month.

Warplanes, believed to be Russian, unleashed a series of airstrikes Sunday on the rebel-controlled district of al-Qatrji in the city of Aleppo, the capital of the province of the same name, the Syrian observatory for Human Rights said.

At least seven civilians were killed.

"The intensity of the Russian air bombing is today as intense as it has been in the past two days," the Observatory's head, Rami Abdel-Rahman, told dpa.

Meanwhile, hostilities were continuing elsewhere on the ground. Turkey shelled US-backed Kurds in northern Syria for the second successive day despite a call for a halt from Washington.

Turkish forces shelled positions held by the Kurd-led Democratic Forces of Syria (DFS), a coalition linked to the Kurdish People's Protection Units (YPG) in the northern and north-western parts of Aleppo province.

The shelling targeted a strategic airbase and other posts that the Kurds had captured earlier this week from Islamist rebels.

Damascus condemned the Turkish action and called on the UN to stop it, Syria's official news agency SANA reported.

On Saturday, Turkey vowed to continue the attack and demanded the Kurds withdraw from the areas. The United States, an ally of Turkey, called on Ankara to stop the shelling and focus on fighting the Islamic State that rules territory in Syria.

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

Saudi Arabia, a staunch critic of al-Assad, has confirmed sending fighter jets used by the US and allied powers to attack Islamic State militants to the Incirlik airbase in southern Turkey.

Saudi Foreign Minister Adel al-Jubeir said Sunday in Riyadh that any decision by his country to deploy ground troops in Syria would be linked to the US-led coalition fighting Islamic State there.

Syria has warned it would fight any troops sent to its territory without its permission.