The United Nations envoy to Syria has launched another attempt to bring the country's warring parties together, saying he will begin meeting their representatives in May.

Staffan de Mistura said he would hold talks in Geneva with the country's government, opposition groups and regional powers including Iran to assess by the end of June whether there is any hope brokering an end to the war.

He briefed the UN Security Council on 24 April on the latest bid to find a political solution to the four-year conflict that has killed more than 220,000 people, displaced an estimated 7.6m and forced nearly 4m to flee the country.

UN Secretary-General Ban Ki-moon asked de Mistura earlier this month to "focus much more to re-launch a political process" after his attempt to broker a local truce in the northern city of Aleppo failed to materialise.

"There is nothing new telling us today that the political process will succeed or not," de Mistura told reporters. "We will start in early May and we will be meeting one after the other, everyone. Not together, separately."

United Nations for Afghanistan
Staffan De Mistura, Special Representative of the United Nations for Syria Reuters

"By the end of June we should hopefully be in the position of reassessing whether there is any convergence on issues of substance or not," de Mistura said, adding that he will report his findings to Ban.

He said Iran would be invited as it was a major player in the region and had influence in Syria. Ban withdrew a last minute invitation to Iran to Syria peace talks in January last year after the Syrian opposition threatened to boycott.

"The U.N. and myself have the right and will be inviting everyone, including Iran," de Mistura said.

Other key world powers would also be consulted, but not the militant groups Islamic State or Nusra Front, which are classified as "terrorist organisations", UN spokesman Ahmad Fawzi said. Some of those present at the talks would be able to communicate with them, he added.

De Mistura described his planned meetings as "a stress test of the willingness to narrow the gaps" three years after the agreement of the Geneva Communiqué, a document setting out guidelines on Syria's path to peace and a political transition.

"There is no excuse for us to wait," he said. "The immensity of the human suffering ... obliges us to seek out even the remotest possibility for some type of change."