UN leader Ban Ki-moon held his first meeting with the head of the opposition Syrian National Coalition in a bid to press for a Syria peace conference.
Saturday's meeting is likely to infuriate President Bashar al-Assad's government, which is still recognised by the United Nations and routinely calls the coalition and other opposition groups "terrorists".
Coalition president Ahmad Jarba told Ban the opposition group was ready to send a delegation to a conference, according to UN spokesman Martin Nesirky.
Ban announced on Friday that he wants to organise in mid-November a follow-up to a Geneva peace conference held in June 2012, when the major powers agreed a declaration that there should be a transitional government in Syria.
"The secretary-general welcomed Mr Jarba's commitment to send a delegation to the Geneva Conference and urged the National Coalition to reach out to other opposition groups and agree on a representative and united delegation," Nesirky said.
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Ban "stressed the importance of embarking on a serious dialogue as soon as possible as well as the need to ensure accountability for war crimes," the spokesman said.
The UN leader said on September 13 that Assad has committed "many crimes against humanity".
He has stepped up criticism of Assad since an August 21 chemical weapons attack near Damascus.
A UN investigation found that banned sarin gas had been used but did not attribute blame.
The US says 1400 people died in the attack and blames Assad's government for the killings. The government and Russia have accused opposition rebels of staging the assault.
The UN Security Council voted on Friday to destroy Syria's chemical weapons in its first resolution on the war since protests against Assad started in March 2011.
The UN says more than 100,000 people have died in the war.
