His comments cames as UN Secretary General Ban Ki-moon 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 in Syria.
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 on Thursday 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."
The inclusion of Iran -- a key backer of Syrian President Bashar al-Assad - in this week's meetings marks a crucial shift after Tehran was excluded from earlier talks, mainly because of opposition from Washington and Riyadh.
The key international players in the conflict remain bitterly divided over the question of Assad.
On one side, Russia and Iran are backing Assad's forces on the ground and say Damascus must be helped to defeat "terrorism" before a political process can take shape.
On the other, the US and its key regional allies Turkey and Saudi Arabia are supporting groups fighting Assad and insist he must step down in favour of a unity government.
Spain's Foreign Minister urged on Thursday for direct negotiations with al-Assad.
"We don't like al-Assad, but Nixon didn't like Mao, and he reached an agreement with him," said García-Margallo during a press conference.
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.