The UN's humanitarian chief heads for Syria on Wednesday to urge the regime to let aid into devastated protest cities, with US President Barack Obama insisting military intervention would be a "mistake".
The five major UN powers discussed on Tuesday new efforts to press for a halt to the violence in Syria, which Obama called heartbreaking, as regime forces pounded rebel towns and the death toll rose.
However Obama cautioned that there was no simple solution in Syria, and warned 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 centres 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.
"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."
Syrian President Bashar al-Assad showed no signs of relenting in his crackdown on anti-regime elements, despite growing diplomatic pressure.
The United Nations says the regime's crackdown has already cost over 7,500 lives in the past year, but Assad vowed to press ahead with his campaign to crush "terrorism."
The United States, championing the diplomatic track, 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 on Tuesday 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.
Several diplomats said that there would be no developments until UN-Arab League special envoy Kofi Annan and UN humanitarian chief Valerie Amos have finished their missions to Syria this week.
Amos is due to arrive in Damascus Wednesday for a two-day stay, in a bid to persuade Assad's government to allow humanitarian aid into protest cities which have been relentlessly bombarded by regime forces.
Annan is to go to Damascus on Saturday to press the humanitarian case and start efforts to persuade Assad to halt the deadly offensive.
Russia's Ambassador Vitaly Churkin did not comment on the international talks, but deputy foreign minister Gennady Gatilov earlier said his country believes the draft resolution is not balanced.
Obama 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.
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."
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 said in Beirut that Syrian forces bombed a bridge used to evacuate the wounded and refugees to Lebanon from Homs province, and fleeing residents 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.
Earlier Tuesday Russia told the West not to expect Moscow to change its stance on the Syria crisis following Vladimir Putin's presidential election victory.
"The Russian position on Syria was never shaped by electoral cycles, in contrast to (that of) some of our Western colleagues," a foreign ministry statement said.
China's former ambassador to Damascus, Li Huaxin, is due in Syria on Wednesday for meetings with the government and other parties.
France's President Nicolas Sarkozy said on Tuesday that he did not regret inviting Assad to Paris for the 2008 Bastille Day parade, despite his later descent into violence.
"He was not a murderer but he has become a murderer," Sarkozy said when asked during a re-election campaign television interview whether he regretted the gesture.