A host of top officials called on the divided UN Security Council to take stronger action to deal with the fallout from the 26 month old civil war in which the United Nations says up to 100,000 people have died.
"The extremely high rate of killings nowadays - approximately 5,000 a month -- demonstrates the drastic deterioration of the conflict," UN assistant secretary general for human rights Ivan Simonovic told a council meeting on Syria.
"We have not seen a refugee outflow escalate at such a frightening rate since the Rwandan genocide almost 20 years ago," Guterres said.
"This crisis has been going on for much longer than anyone feared with unbearable humanitarian consequences," he added.
UN humanitarian chief Valerie Amos said the international community may have to consider cross border operations to get aid into Syria.
Syria's UN ambassador Bashar Jaafari disputed the UN death toll figures as "unprofessionally sourced.