Syria's main opposition bloc comprising political and military opponents of President Bashar al-Assad is reportedly considering an offer made by United Nations special envoy Staffan de Mistura. This would aid starting the negotiations to end the civil war, which sparked off five years ago in the Middle Eastern country.

"De Mistura made them a proposition, and that's tempting them to enter the negotiations. They are very prudent," Reuters quoted Western diplomatic source as saying.

Syrian opposition group, High National Committee (HNC), had earlier threatened to walk out of talks if shelling in opposition-held areas do not end and if humanitarian aid does not reach the besieged towns. It has also demanded the release of 3,000 women and children from government jails. 

"If the regime insists on continuing to commit these crimes, then the HNC delegation's presence in Geneva will not be justified... The delegation will inform de Mistura of its intentions to withdraw its negotiating team if the UN and world powers are unable to stop these violations," Al Jazeera quoted HNC coordinator Riad Hijab as saying.

However, HNC said the Sunday talks with de Mistura were positive and encouraging "concerning humanitarian issues".

According to the Reuters' source, HNC "want tangible and visible things immediately. Things they can give to their grassroots... Certain things aren't possible immediately like the end of the bombings, but the easiest is the release of civilians, women and children".

De Mistura will again hold "proximity talks" with HNC and Syrian government delegations on Monday that is expected to pave way for six-month-long negotiations to end the civil war and bring peace to the nation.

Bashar al-Jaafari, the Syrian government's delegation head in Geneva, said Damascus was considering ceasefires, humanitarian corridors and prisoner releases after successful talks. 

Over 2.5 lakh people have been killed and more than 10 million have migrated to safer countries for asylum since the war started in 2011.