UN leader Ban Ki-moon has said that the UN Security Council's failure to agree a resolution on Syria has been "disastrous" for the country's people because of the deaths since the vote.
"It has encouraged the Syrian government to step up its war on its own people," Mr Ban said.
He was speaking to reporters after briefing the UN Security Council about events in the Middle East.
Russia and China vetoed a UN security resolution for Syrian President Bashar al-Assad to resign at the weekend.
Forces loyal to President Assad have killed at least 47 civilians in the latest attacks on the Syrian city of Homs, activists said.
The troops were reported to have fired rockets and mortar rounds to subdue opposition districts, a day after Russia said Mr Assad wants peace.
Tanks entered the Inshaat neighbourhood and moved closer to Bab Amro district in the central Syrian city, which has been the target of the heaviest barrages by loyalist troops.
They have killed at least 150 people in the last two days, activists in the city and opposition sources said.
An activist there said bombardment intensified in the early morning, concentrating on Bab Amro, al-Bayada, al-Khalidiya and Wadi al-Arab.
They are all Sunni Muslim neighbourhoods in the mixed city that have risen up against the 11-year rule of President Assad.
The official state news agency said "armed terrorist groups" attacked police roadblocks in Homs and fired mortar bombs at the city, with three falling on the Homs oil refinery, one of two in the country.
The reports could not be independently verified because Syrian authorities have placed tight restrictions on access to the country by Western media.
Elsewhere, the US says it is looking to build international consensus about what steps to take to solve the growing political and humanitarian crisis in Syria.
"In the coming days we will continue our very active discussions ... to crystallise the international community's next steps in that effort to halt the slaughter of the Syrian people," White House Press Secretary Jay Carney told reporters.
Mr Carney said the discussions, which would include the opposition Syrian national council, were aimed at helping the process "move toward a peaceful, political transition, democratic transition in Syria," but gave no details.
The White House continued to stress it was not actively considering military intervention to prevent a crackdown on opponents of Mr Assad's rule in which thousands have been killed.
"We never rule anything out in a situation like this. But we are pursuing a path that includes isolating and pressuring the Assad regime so that it stops its heinous slaughtering of its own people," Mr Carney said.