UN Secretary General Ban Ki-moon on Thursday urged participants at international talks aimed at resolving the brutal war in Syria to show "flexibility" and welcomed Iran’s participation for the first time in the discussions.

UN Secretary General Ban Ki-moon on Thursday urged participants at international talks aimed at resolving the brutal war in Syria to show "flexibility" and welcomed Iran's participation for the first time in the discussions.

Iranian Foreign Minister Javad Zarif will join key players in the Syria crisis -- including Russia, the United States and Saudi Arabia -- in Vienna on Friday for a renewed effort to end more than four years of bloodshed.

It will be the first time all the major players in the conflict are in the same room, though there has been no mention of either the Syrian government or the opposition attending.

"I am encouraged that the high level leaders are meeting in Vienna tomorrow to discuss the situation in Syria," the UN chief told a news conference in Madrid when asked about Iran's participation the talks.

"My sincere hope is that they will really address this issue with a sense of flexibility, whatever differences they may have in their political views, in their approaches. They should be united."

Ban urged the five "critically important" nations that will take part in the talks -- Iran, Russia, Saudi Arabia, Turkey and the United States -- to "show their global leadership rather than their own national perspectives".

"The longer they take their own national perspectives, much more people will suffer and the hold world will suffer," he added.

"As I always say, there is no military solution," Ban said.