Ban Ki-moon has told Syria's prime minister at the Non-Aligned Movement (NAM) summit in Tehran that fighting must stop in Syria "with the primary responsibility resting on the government to halt its use of heavy weapons".
In a meeting on Friday with Wael al-Halaqi and Walid Muallem, Syria's foreign minister, the UN secretary-general said he set out his "demands for all sides to cease all forms of violence".
"What is important at this time is that all the parties must stop the violence. All those actors who may be providing arms to both sides... must stop," Ban said at a news conference broadcast live on Iranian television.
Ban held his meeting with Halaqi on the sidelines of the two-day summit that ends on Friday.
He said he had asked Iran to support his call on Syria, "and I have a strong assurance from Iran that it will do so".
Al Jazeera's Imran Khan at the summit said NAM foreign ministers had agreed on a draft statement to end the crisis in Syria, which included opposing foreign intervention and welcoming UN-Arab League envoy Lakhdar Brahimi.
However, Khan said that statement was never made public and wasn't mentioned after a closed-door meeting of heads of state later in the day on Friday.
"This conference was never supposed to be about Syria … it’s about coming up with resolutions on developing countries," Khan said.
"However, Syria has been the dominant talking point."
Ban also said he had a series of meetings with Brahimi, who has taken over from Kofi Annan in spearheading international efforts to broker peace in Syria.
In-depth coverage of escalating violence across Syria
On Thursday the Egyptian President Mohamed Morsi said it was an "ethical duty" to support the Syrian people against the "oppressive regime" in Damascus.
His comments sparked a walkout by the Syrian delegation.
The conflict in Syria has claimed more than 26,000 lives since it began in March 2011, according to the Syrian Observatory for Human Rights, a UK-based actvist group.
Iran, the principal ally of Syrian President Bashar al-Assad, has accused the US, Saudi Arabia, Qatar and Turkey of arming Syria's rebels.
The Syrian opposition and US officials in turn allege that Iran is giving military help to Assad.