Syria's UN ambassador Bashar al-Jaafari on Tuesday said the third round of indirect talks aimed at fostering a political solution to the conflict in Syria was "useful and constructive," reported the official Syrian Arab News Agency.

Bashar al-Jaafari accused "terrorism-sponsoring countries and the terrorist groups that belong to them" of undermining efforts to establish a political solution.

They "seek to undermine the political solution efforts through breaching the cessation of hostilities agreement and carrying out terrorist acts that claimed dozens of innocent lives," al-Jaafari told reporters in Geneva.

Last week, the Saudi-backed mainstream opposition alliance High Negotiations Committee (HNC) suspended its participation in the talks, citing continued attacks on civilians by government forces and stalled aid deliveries.

"It is not suitable, neither morally nor on the humanitarian side, to be part of negotiations when Syrians are dying daily from sieges, hunger, bombings, poisonous gases and barrel bombs," said former Syrian Prime Minister Riad Hijab, head of the HNC.

Aleppo under siege

At least 11 civilians, including two children, were killed during an aerial bombardment of the northern Syrian city of Aleppo on Tuesday, reported the UK-based Syrian Observatory for Human Rights (SOHR), which relies on a network of informants on the ground.

The White Helmets, a volunteer civil defense group in Syria, reported that five of their rescue workers were killed in regime bombardment.

After speaking with the HNC's Hijab, British Foreign Minister Philip Hammond called for government forces to end their bombardment of Aleppo, where more than 50 civilians have been killed since last week.

I am "deeply concerned by Syrian regime bombing of civilians and military escalation in Aleppo. Violations of cessation of hostilities must stop," Hammond wrote in a tweet.

A US-Russia brokered ceasefire that went into effect on February 27 has witnessed a significant increase violations.

More than 270,000 Syrians have been killed and roughly half the population displaced since the conflict erupted in 2011, when government forces violently quashed pro-democracy protests calling for President Bashar al-Assad's ouster.