International powers are "in a race against time" to prevent all-out civil war in Syria, where the government could use the presence of ceasefire observers to prepare a new assault, UN leader Ban Ki-moon warned.
Speaking ahead of a key UN Security Council meeting on Syria on Tuesday, Ban again condemned the "brutality" of President Bashar al-Assad's forces but said attacks by opposition groups have also "escalated."
"We are in a race against time to prevent full-scale civil war -- death on a potentially massive scale," Ban said. The UN already estimates that well over 9,000 people have died in the 14-month uprising against Assad.
"The government continues to assault its people," the UN secretary-general told the Atlantic Council think-tank in Washington.
"Every day we see the most appalling images -- troops firing in city centers, innocent civilians dying, even children. Security forces are arresting and torturing people with great brutality."
A ceasefire started on April 12 under an accord between Assad and UN-Arab League envoy Kofi Annan has broken down, with fighting raging on between government forces and anti-Assad rebels.
There are now about 60 UN ceasefire observers in Syria and the full force of 300 sent by the UN Security Council is expected to be in place before the end of May, Ban added.
"This is a difficult mission at a difficult time," he said.
"We know the security risks to our brave UN observers. We know that Syrian citizens could face punishment for even speaking with them. And we know the nature of the regime, which could well use the presence of the mission to prepare further violence."
Ban said Assad's government must arry out Annan's six-point peace plan, including the withdrawal of troops and guns from cities, "without further delay."
"We cannot predict how this will end," he said. "But we do know there can be no compromise on fundamental principles of justice and human rights."
Ban earlier slammed the Syrian government for holding a national election on Monday despite the ongoing violence and for failing to involve all parties.
"Only a comprehensive and inclusive political dialogue can lead to a genuine democratic future in Syria," the UN chief said in a statement released by his office. "These elections are not taking place within that framework."
Annan is due to brief the Security Council on Tuesday, from 1400 GMT, on his struggle to establish the cessation of hostilities and get a political dialogue started.
Annan and Ban have said there is evidence that observers have reduced violence in places where they are present.
But the United States has warned it could call for the UN observer mission to be halted before the end of its 90-day mandate. Other western nations also have doubts that the Assad government will respect the envoy's peace plan.
"We will be carefully looking for signs that Annan can see the peace plan is cracking," one western diplomat said.
Another UN diplomat said it was "difficult to see whether the opposition will step up for talks when they are being shot, shelled and tortured."