The five major UN powers discussed on Tuesday new efforts to press for a halt to the violence in Syria, which US President Barack Obama called heartbreaking, as regime forces pounded rebel towns and the death toll rose.
But Syrian President Bashar al-Assad showed no signs of relenting in his crackdown on anti-regime elements that the United Nations says has cost more than 7,500 lives in the past year and vowed to press ahead with his campaign to crush "terrorism."
The United States is leading work on a text for the badly divided UN Security Council, where Russia and China have twice used their powers as permanent members to veto Syria resolutions.
A new draft obtained by AFP calls on the Syrian government to immediately cease all violence, withdraw security forces from protest cities and release prisoners held over the protests.
It then calls on the opposition to "refrain from all violence" once these conditions are met.
Ambassadors from permanent members Britain, China, France, Russia and the United States held talks along with the envoy from Morocco, the current Arab member of the council, but none said a vote was expected.
"I don't think you should expect anything specifically," US Ambassador Susan Rice told reporters.
Russia's Ambassador Vitaly Churkin did not comment, but Deputy Foreign Minister Gennady Gatilov earlier said his country believes the resolution is not balanced.
Several diplomats said there would be no developments on the resolution until UN-Arab League special envoy Kofi Annan and UN humanitarian chief Valerie Amos have finished missions to Syria this week.
Obama cautioned that there was no simple solution in Syria, warning that unilateral military action would be a mistake.
He was speaking after top Republican Senator John McCain called for US air strikes on Syrian forces to protect population centers and create safe havens for opponents of the regime.
"What's happening in Syria is heartbreaking, and outrageous, and what you've seen is the international community mobilise against the Assad regime," Obama told a White House press conference.
Assad has "lost legitimacy of his people. And the actions that he is now taking against his own people is inexcusable," the president said.
"On the other hand, for us to take military action, unilaterally, as some have suggested, or to think that somehow there's some simple solution, I think is a mistake."
He cautioned that the situation was not the same as in Libya, when the United States used its air force to back a NATO no-fly zone.
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There, the United States "had the full cooperation of the region, Arab states, and we knew that we could execute very effectively in a relatively short period of time. This is a much more complicated situation."
Obama's cautionary remarks were backed up by a top US general, who said military intervention in Syria would be "extremely challenging" given the country's advanced air defenses and the presence of Al-Qaeda extremists.
Syria has Russian-made missiles and radars that would complicate any outside effort to use air power to protect civilians or resistance forces, General James Mattis, head of US Central Command, told the Senate Armed Services Committee.
"The Russians have provided very advanced integrated air defense capabilities -- missiles, radars, that sort of thing -- that would make imposition of any no-fly zone challenging if we were to go that direction," Mattis said.
Asked about Iran's role, Mattis said Tehran was working hard to prop up Assad and had flown in advisers, weapons and equipment, including eavesdropping devices.
He said the eventual fall of Assad's regime would deal a damaging blow to Iran, which relies on its alliance with Damascus to bolster Hezbollah militants in Lebanon.
"It'll be the biggest strategic setback for Iran in 20 years. When Assad falls, not if, but when he is going to go," Mattis said.
Against that backdrop, Assad said "the Syrian people ... have again proven their capacity to defend the nation and to build a new Syria through their determination to pursue reforms along with the fight against foreign-backed terrorism," he said, quoted by state news agency SANA.
At least 16 people were killed on Tuesday as Syrian forces launched a major assault on Herak, a town in the southern province of Daraa, a monitoring group said.
"Large military forces, including tanks and armoured troop carriers, launched an assault on Herak," the Britain-based monitoring group added, citing residents.
After fleeing the battered Baba Amr district in the flashpoint central city of Homs on Thursday, rebels regrouped in nearby Rastan, which has been bombed intermittently since February 5 and which the Observatory and activists said came under artillery fire on Sunday and Monday.
Qusayr, another town in Homs province that has fallen mainly under rebel control, was also targeted by heavy bombardment, according to Anas Abu Ali, an official with the FSA.
Observatory chief Rami Abdel Rahman told AFP in Beirut that Syrian forces bombed a bridge used to evacuate the wounded and refugees to Lebanon from Homs province, and fleeing refugees have given terrifying accounts of atrocities committed by government troops.
The International Committee of the Red Cross, meanwhile, was negotiating with Syria authorities for a fourth day to be allowed to deliver aid and evacuate the wounded from Baba Amr.
Russian Foreign Minister Sergei Lavrov said he was to meet his Arab counterparts on Saturday in Cairo to discuss Moscow's ally Syria.
And China's former ambassador to Damascus, Li Huaxin, is due in Syria on Wednesday for meetings with the government and other parties.