A possible spillover of the Syrian crisis into neighboring countries must be addressed "to prevent the flames of war from spreading across the Middle East," Guterres said, according to a UNHCR statement.
Guterres and Lebanese Prime Minister Najib Mikati appealed to international donors for $1.7 billion in aid.
The number of Syrians fleeing to Lebanon is projected to exceed 1 million by the end of the year.
"There is not a village, city or town in Lebanon that is not hosting Syrian refugees," Guterres told reporters in Beirut.
"The international community must overcome its divisions and come together to stop the fighting if we want to prevent the flames of war from spreading across the Middle East," Guterres said.
Iran's semiofficial news agency Mehr reported Tuesday that a Foreign Ministry spokesman has denied a report first published Sunday in British newspaper The Independent that Iran has decided to send 4,000 troops to support Syria's government.
"These news media reports are pure lies and unrealistic fabrications," said Abbas Araqchi, the spokesman. "Iran has never and will never send Iranian forces to Damascus."
U.S. officials and Syrian opposition fighters say they have intelligence that Iranian forces are in Syria.