The Syrian government has responded with "constructive interest" to the UN proposal to suspend fighting in the city of Aleppo, according to Staffan de Mistura, the UN special envoy.

His public call for a ceasefire came a day after Syrian President Bashar al-Assad said the suggestion was "worth studying".

"My meetings here with the government and with President Bashar al- Assad gave me the feeling that they are studying very seriously and very actively the UN proposal," De Mistura said in Damascus on Tuesday.

De Mistura, at the beginning of his three-day visit to Syria, said the UN was now waiting for contact with the other stake-holders.

There has been no response from groups on the other side of the conflict, who remain an important player in the "action plan" put forward by De Mistura.

Assad said on Monday that he was ready to study the UN plan to freeze fighting in Aleppo, a city divided since the major offensive in mid-2012 between government and rebel control.

De Mistura set out the plan last month that would allow humanitarian aid through and will lay the groundwork for peace talks.

In the plan he called Aleppo a "good candidate" for such a freeze in the fighting.

"All Syrians need a concrete example. ... That's why we have come to the conclusion of making a specific proposal." he said.

Historical significance

De Mistura stressed that the plan is not yet a peace plan but could be an incentive to move in that direction.

Aleppo was chosen because of its importance as Syria's second city. It was once the country's industrial powerhouse, and a place of cultural and historical significance.

"Aleppo city is not far from possible collapse and we need to do something before that happens. People in Aleppo are tired, everybody runs all day just to find bread.They only have food aid, donated baskets,'' De Mistura said.

Aleppo is also the last major city where rebels hold large areas, but they are under attack from advancing government forces.

The Islamic State in Iraq and the Levant (ISIL) has been trying to take nearby rebel-held communities and activists fear they will try and advance on to Aleppo.

Many activists said the UN's efforts may be too late because of the damage done during three years of war.

But Mohammed al-Shafi, an activist from Aleppo, said if the government agreed to a truce, residents would pressure rebels to follow suit.

More than 195,000 people have been killed in Syria since the beginning of the conflict in March 2011, several internationally backed negotiations were attempted but have failed to bring the rival sides to an agreement.

Against this backdrop, Sheikh Tamim bin Hamad Al Thani, the ruler of Qatar, an important US ally, cautioned on Tuesday that the ongoing air strikes against ISIL were not enough to defeat "terrorism and extremism" in Iraq and Syria.

He urged the international community to take action against the factors fuelling radicalism there.

He specifically blamed the policies of Assad and "some militias in Iraq" - in an indirect reference to Iranian-backed armed Shia groups - for the rise of ISIL.