President Barack Obama said it was only a matter of time before Syrian President Bashar al-Assad left office, but squarely opposed a call to launch U.S. military action to force him out.

Obama said what was happening in Syria was heartbreaking and outrageous, and witness accounts of the devastation after government troops bombarded the rebel stronghold of Baba Amr into submission have given attempts to reach a diplomatic solution renewed urgency.

The atrocities we saw were beyond our imagination, said one former resident, speaking from a secret location.

U.N. humanitarian chief Valerie Amos and the joint U.N.-Arab League envoy Kofi Annan are due to visit Syria this week to see firsthand the effects of a conflict that the United Nations says has killed more than 7,500 civilians.

World powers met behind closed doors at the United Nations on Tuesday to discuss a U.S.-drafted resolution urging an end to the crackdown on the revolt against Assad and unhindered humanitarian access.

But despite the chorus of outrage, Western leaders have ruled out a Libya-style military intervention in Syria, fearing it could trigger wider conflict in the Middle East.

The White House said Obama was committed to diplomatic efforts to end the violence, saying Washington wanted to isolate Assad, cut off his sources of revenue and encourage unity among his opponents.

Ultimately this dictator will fall, Obama said at a news conference in Washington on Tuesday, adding that it was not a question of if, but when Assad would be forced out.

But he opposed a call by U.S. Senator John McCain, who lost to Obama in the 2008 presidential election, for the United States to lead an international effort to protect population centres in Syria with air strikes on Assad's forces.

For us to take military action unilaterally, as some have suggested, or to think that somehow there is some simple solution, I think is a mistake, he said.

SMELL OF DEATH

Obama's comments came as Assad faced growing Western anger for preventing aid from entering the Baba Amr district of Homs, and over accusations of rights abuses, including television images said to show torture victims at a hospital in the city.

The Red Cross said it was still waiting for approval to distribute aid to Baba Amr but residents who had fled across the border to Lebanon told of the destruction.

The smell of death was everywhere. We could smell the bodies buried under the rubble all the time, said Ahmad, who escaped to Lebanon. We saw so much death that at the end the sight of a dismembered body ... stopped moving us.

Syrian state television aired footage of dozens of men, women and children returning on foot to Baba Amr on Tuesday, passing bullet-pocked and damaged buildings, days after the rebel fighters fled nearly a month of shelling.

It showed pictures of rocket-propelled grenades and guns laid out on the street. Assad's government says the uprising is a campaign by foreign-backed Islamist insurgents that has killed 2,000 police and soldiers since the protests erupted in March.

Secretly shot video footage aired on Monday by a British television station showed what it said were Syrian patients tortured by medical staff at a state-run hospital in Homs.

The video, which Channel 4 said it could not independently verify, showed wounded, blindfolded men chained to beds. Most foreign journalists are banned from Syria.

The U.N. torture investigator, Juan Mendez, said the Channel 4 clips appeared to support increasingly grave allegations pointing to crimes against humanity.

In Homs, activists said security forces were carrying out raids in a district next to Baba Amr on Tuesday, and reported gunfire and explosions in another area.

In Herak, in Deraa province where the revolt erupted nearly a year ago, residents said armoured vehicles and tanks had massed on the western fringe of the city and in parts of the centre. There were raids reported in the city of Deir al-Zor.

NEW MOVE AT UNITED NATIONS

Calls for action to protect civilians have grown louder as the Alawite-led security apparatus has cracked down on the uprising that has its roots in the majority Sunni community and which has raised the prospect of a civil war.

Turkish Prime Minister Tayyip Erdogan, a former ally of Assad, said the violence in Syria had started to resemble an inhumane savagery in recent days, calling for a humanitarian corridor to be established in Syria to help civilians.

The five permanent members of the U.N. Security Council and Morocco held what U.S. Ambassador Susan Rice called preliminary discussions ... about whether there is any possibility of reaching agreement on a U.S.-drafted resolution.

The draft, obtained by Reuters, demands unhindered humanitarian access and condemns the continued widespread, systematic, and gross violations of human rights and fundamental freedoms by the Syrian authorities and demands that the Syrian government immediately put an end to such violations.

Several Western diplomats expressed disappointment with the draft, saying it fell far short of an appropriately stinging condemnation of the Syrian government.

It remains unclear whether the draft, which follows two proposed condemnations of Damascus that Russia and China vetoed, has any chance of success in the 15-nation council, which has been deadlocked over Syria for almost a year.

A Chinese diplomat arrived in Damascus on Tuesday to outline Beijing's peace plan.

U.N. ambassadors from the United States, Britain, France, China and Russia, along with the sole Arab state on the 15-nation Security Council, Morocco, avoided detailed comments when they left a 1-1/2 hour meeting on the draft on Tuesday.

Chinese Ambassador Li Baodong told reporters: We're still working on that.

(Additional reporting by Oliver Holmes and Mariam Karouny in Beirut, Suleiman al-Khalidi and Khaled Yacoub Oweis in Amman and Louis Charbonneau at the United Nations; Editing by Michael Roddy and Alison Williams)