The UN’s envoy on Syria, Staffan de Mistura, held meetings this week aimed at reviving peace talks, a statement said Tuesday, amid mounting pressure to end the four-year war.

The UN's envoy on Syria, Staffan de Mistura, held meetings this week aimed at reviving peace talks, a statement said Tuesday, amid mounting pressure to end the four-year war.

After several attempts to find a political solution, De Mistura in July proposed a fresh approach whereby Syrians would take part in "thematic" working groups.

The envoy has met with the heads of these groups over the last two days, with the aim that this will "set the stage for a Syrian agreement to end the conflict," De Mistura's office said in a statement.

The working groups focus on safety and protection; political and legal issues; military, security and counter-terrorism; and public services and reconstruction.

De Mistura hopes the UN-appointed heads of the groups can lay the groundwork before representatives of the various factions in Syria join the talks.

But Syrian President Bashar al-Assad's regime has reportedly warned him the outcomes of the working groups cannot be binding.

The crisis in Syria has killed nearly a quarter of a million people since 2011 and forced more than four million to flee the country. Millions more have been displaced within Syria.

Hundreds of thousands of Syrian refugees have reached Greece this year, fuelling Europe's migrant crisis and leading to growing calls for a political solution to end the fighting.

"This is the defining humanitarian challenge of our times," De Mistura said. "The Syrians deserve that we move faster towards a political solution".