The European Union has welcomed Syrian President Bashar al-Assad's decision to allow humanitarian access to the town of Madaya, where people are said to be dying from starvation, and called for a halt to all attacks on civilians in the conflict, ahead of peace talks later this month.

The war in Syria has killed an estimated quarter of a million people in nearly five years, ravaging the country and creating a breeding ground for radical Islamists as regional allies and global players back different sides in the conflict.

The UN hopes to convene talks between Damascus and the Syrian opposition on January 25, and the blockade of Madaya, near the border with Lebanon, has become a focal issue for Assad's opponents.

"The decision of the Syrian regime to allow humanitarian access in Madaya is a first step in the right direction," Federica Mogherini, the EU's foreign policy chief, and the bloc's Commissioner for Humanitarian aid and Crisis Management, Christos Stylianides, said in a joint statement.

"The EU welcomes it and expects it will be fully implemented and extended by all parties to all the cities under siege."

A new Syrian opposition group created to oversee peace negotiations has also demanded Damascus halt the bombardment of civilian areas and use of barrel bombs, and urged it to release detainees before the talks, calls echoed by Ms Mogherini and Mr Stylianides.

"It will be important to implement concrete confidence building measures in support of the upcoming intra-Syrian political talks scheduled to start at the end of January: an end to attacks on civilians, to aerial bombardments and sieges of civilian areas," they said in the EU statement.