The West clashed with Russia at the United Nations Security Council over Syria, as activists and the Damascus government traded blame for a massacre of civilians in the city of Homs.
The conflict appeared to inch closer to civil war with the exiled Syrian National Council (SNC) saying it was preparing to arm anti-government rebels with foreign help. But the opposition to President Bashar al-Assad remained fragmented.
SNC representatives were to meet at 0800 GMT on Tuesday in Ankara with U.N.-Arab League envoy Kofi Annan, who held talks with Assad on Saturday and Sunday.
In Rankous, close to Damascus, army bombardment turned into clashes after dark on Monday. Activists said there were sounds of explosions and casualties. Rebels said they killed dozens of troops in an ambush plotted with defectors still in army ranks.
There were no signs after Monday's special Security Council meeting on the Arab Spring uprisings that the five permanent members were any closer to breaking an impasse that has twice led Russia and China to veto draft resolutions on Syria.
U.S. Secretary of State Hillary Clinton told reporters after meeting privately with Russian Foreign Minister Sergei Lavrov it was time for Moscow and Beijing to join calls for an end to the Syrian government's repression of unrest.
Clinton said it was up to Assad, whose army has led a year-long crackdown which the United Nations says has killed well over 7,500 civilians, to take the initial step. First and foremost the Assad government has to end the violence, she said.
Lavrov said NATO had grossly violated its U.N. mandate to protect civilians in Libya last year after rebels rose up to overthrow and kill Muammar Gaddafi.
There is no doubt whatsoever that the Syrian authorities bear a huge responsibility for the current situation, he said.
But, Lavrov added, there was no point in talking about who started the violence. He said the Security Council should press for an immediate end of violence by all sides.
Both Assad's government and the rebel side agreed that in Homs dozens of killings in cold blood were carried out on a weekend when Annan was visiting Syria to seek a ceasefire, humanitarian access and dialogue.
Activists in Homs and state television showed videos of bloodied bodies with hands tied behind their backs.
Opposition activists said militants loyal to Assad killed over 50 people in a district of Homs called Karm al-Zeitoun.
I saw two females who were raped, one was around 12 or 13 years old. She was covered in blood and her underclothes were off, said a medic in Homs.
One of the women was strangled; she had bruises on her neck. Some of the bodies I saw, especially the children, had their throats slit.
Government restrictions on media access made it hard to assess conflicting reports of the mass killings.
State media said militants committed the killings to influence the U.N. Security Council's meeting on Monday.
The terrorist armed groups have kidnapped scores of civilians in Homs, killed and mutilated their corpses and filmed them to be shown by media outlets, state news agency SANA said.
ARMS FOR REBELS
A spokesman for the Syrian National Council called on foreign powers to intervene and said the opposition group had already set up a bureau to send arms to the rebels with the help of foreign governments. He would not name the countries or the location of the bureau.
Sunni Muslim Saudi Arabia and Qatar have called for the rebels against Assad, who is allied with Shi'ite Iran, to be armed.
We demand military intervention by Arab and Western countries to protect civilians, George Sabra of the SNC told reporters in Istanbul.
We demand establishment of secured humanitarian corridors and zones to protect the civilians. We demand implementation of a no-fly zone over entire Syria to prevent Assad from continuing massacres.
In Cairo, the Syrian Media Services opposition group said it had started a sit-in outside the office there of the SNC, accusing its leadership of corruption and not listening to calls for reform.
U.N. investigators said on Monday that Syrian forces had used collective punishment against civilians and were accused of carrying out executions and mass arrests in the Baba Amr district of Homs.
Syria's ambassador to the U.N. in Geneva rejected the panel's work as politicised and said al Qaeda fighters from 13 different countries had infiltrated Syria.
(Editing by Andrew Roche)