UN chief Ban Ki-moon says the Arab League plans to revive its mission to Syria, and has sought help from the UN.
Mr Ban said the "disastrous" failure to agree a UN resolution on Syria had encouraged Damascus "to step up its war on its own people".
The international community is struggling to find a way forward in the crisis after Russia and China blocked the resolution last week.
Syrian forces are continuing to attack opposition strongholds in the country.
Hundreds of people have died in the central city of Homs after days of heavy bombardment.
Speaking at the UN, Mr Ban said: "I fear that the appalling brutality we are witnessing in Homs, with heavy weapons firing into civilian neighbourhoods, is a grim harbinger of worse to come."
He said Arab League chief Nabil el-Arabi had told him he intended to revive a monitoring mission in Syria, which collapsed last month amid escalating violence.
Mr Ban said Mr Arabi had asked for UN help with the project and proposed a joint UN-Arab League observer mission, including a joint special envoy.
"In the coming days we will further consult with the council before fleshing out details,'' Mr Ban said.
"We stand ready to assist in any way that will contribute toward improvement on the ground.''
Earlier, the US rejected Russian calls for talks between the Syrian government and the opposition.
White House spokesman Jay Carney said President Bashar al-Assad had missed the "opportunity" for dialogue.
"From the (earliest) days of this situation in Syria, there was an opportunity for the Assad regime to engage in dialogue with the opposition," White House spokesman Jay Carney said.
"Rather than take that opportunity, Assad brutally cracked down on his own people. We don't think that that opportunity is available any more.
"It is clear that Assad has chosen a path and that choice has resulted in the deaths of many Syrians, including innocent children."
Russian Foreign Minister Sergei Lavrov, who visited Damascus on Tuesday, had said that President Assad was ready to talk with "all political forces" to end the violence.
But his comments were dismissed by opposition leaders and violence across Syria has continued unabated.
Mr Carney said the US planned to meet soon with its international partners to discuss ways to stop the violence in Syria and provide humanitarian aid.
In a day of intense diplomatic activity, Turkish Foreign Minister Ahmet Davutoglu said on Wednesday that if the UN Security Council couldn't protect Syrian civilians, then like-minded countries should find ways to do so.
He is due in Washington for talks with Secretary of State Hillary Clinton early next week.
Russian President Dmitry Medvedev, in a phone conversation with French President Nicolas Sarkozy on Wednesday, urged the West "to avoid hasty unilateral steps" in Syria, Russian news agency Interfax reported.
Meanwhile, the UK-based Syrian Observatory for Human Rights said more than 50 people were killed in Wednesday's shelling of Homs.
Witnesses in the Baba Amr neighbourhood reported intense shelling by tanks, mortars, artillery and heavy machine guns.
"The situation is dire. We are short of food, water and medical aid. Doctors have collapsed after treating the wounded without rest for five days," said activist Omar Shaker in Homs.
"We want Lavrov to come and spend a night in Homs to see what we have been passing through."
The Observatory also reported at least another eight civilians killed around the country on Wednesday.
Syria restricts access to foreign media and casualty figures cannot be independently verified.
The UN resolution backed an Arab League peace plan that would have seen President Assad hand power to a deputy to oversee a transition.
Russia, a strong ally of Syria, said that amounted to regime change.
The Syrian government blames the violence on foreign-backed groups and insists the Homs offensive will continue until "order" is restored in the city.