President Barack Obama said on Tuesday it was only a matter of time before Syria's President Bashar al-Assad left office, but added it was a mistake to think the U.S. could take unilateral action there.

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

But he squarely opposed a call by U.S. Senator John McCain who on Monday urged U.S. air strikes on Assad's forces.

McCain, an influential Republican who lost to Obama in the 2008 presidential election, said the United States should lead an international effort to protect Syrian cities and towns.

Obama said it was a mistake to think there was a simple solution to the now year-long crackdown on the opposition in Syria, or that the United States could act unilaterally.

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

Dozens of men, women and children returned on foot to Baba Amr, state television said, passing bullet-pocked and damaged buildings, days after rebel fighters pulled out after a sustained and heavy military assault.

The Red Cross was awaiting approval to distribute aid to the devastated district which endured a month of siege.

Residents who fled the district spoke of bodies decomposing under rubble, sewage mixing with litter in the streets, and a campaign of arrests and executions.

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.

Despite their chorus of outrage as Homs residents gave more detailed accounts of the siege of Baba Amr, 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 on Tuesday that Obama was committed to diplomatic efforts to end the violence, saying Washington sought to isolate Assad, cut off his sources of revenue and encourage unity among his opponents.

But calls for action to protect civilians have grown louder as the Alawite-led security apparatus cracked down on protests and an 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's, 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.

GUNFIRE AND EXPLOSIONS

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.

A Chinese diplomat arrived in Damascus on Tuesday to outline Beijing's peace plan, while U.N. envoys Kofi Annan and Valerie Amos are expected in the Syrian capital this week.

The United Nations says more than 7,500 civilians have died in Syria's bloody crackdown on protests against Assad's government, which says the nearly year-long uprising is a campaign by foreign-backed Islamist insurgents.

Authorities said in December 2,000 police and soldiers had been killed since protests, inspired by Arab uprisings which have overthrown four veteran leaders, erupted last 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. A rubber whip and electrical cable lay on a table in one ward. Patients showed what looked like signs of severe beatings.

Channel 4 said the footage was filmed covertly by an employee at the Homs military hospital and smuggled out by a French photo-journalist identified only as Mani.

The United Nations said it has similar footage, and the clips shown by the British station appear to support increasingly grave allegations pointing to crimes against humanity, the U.N. torture investigator said.

Juan Mendez, United Nations special rapporteur on torture worldwide, said that while he had not seen the Channel 4 video, it seemed in line with recent reports he has received on Syrian forces torturing opponents.

Unfortunately this new allegation is consistent with what my mandate (office) has been receiving over the last several months. The new allegation only adds to the gravity of the situation, Mendez told Reuters in Geneva.

THE END WAS NEAR

It was not immediately clear whether U.N. humanitarian affairs chief Amos would have the unhindered access she has been demanding. Several Western diplomats told Reuters privately they were concerned Damascus appeared to have waited until it had finished the job of punishing Homs before allowing Amos in.

Residents of Baba Amr said they knew the end was near when the army blew up a 3-km (2-mile) tunnel they had used to smuggle in essentials keeping them alive.

Syrian state television showed pictures of rocket-propelled grenades and guns laid out on the street, weapons the presenter said belonged to armed terrorists.

This is the tunnel used by the terrorists to get weapons inand out, the presenter said, standing in a school building next to a 2-m (10-foot) hole in the ground.

A man who fled Baba Amr a day after the army went in said soldiers raided houses, arresting men and executed some of them. Activists say at least 60 men were executed since Friday.

They are cleansing the neighbourhood, they are robbing houses, arresting people then executing some. Baba Amr is besieged from all sides. It is a disaster, said Omar, speaking by telephone from inside Homs after he fled Baba Amr.

They said they have a list of 1,500 men and they want them all ... They are shooting everything that is moving, even animals, he said with a trembling voice.

(Additional reporting by Oliver Holmes and Mariam Karouny in Beirut, Suleiman al-Khalidi and Khaled Yacoub Oweis in Amman; Editing by Michael Roddy)

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