abandoned

"The spectre of an all-out war, with an alarming sectarian dimension, grows by the day," Annan told a meeting of members of the Arab League, co-sponsor with the United Nations of a peace plan aimed at ending the killing in Syria.

The concerns came as Syria was convulsed by yet more fighting yesterday as heavy clashes took place between rebel and regime forces around the town of Deraa and on the outskirts of the capital, Damascus.

The British-based Syrian Observatory for Human Rights said in a statement that two civilians were also killed, one during army raids in Damascus and one by gunfire in the central city of Homs, where activists say the army has been waging a shelling offensive on opposition districts.

Yesterday, Annan gave a bleak assessment of the situation in Syria 15 months on from the start of the anti-Assad uprising. He conceded that efforts by the UN and the Arab League to make a ceasefire take hold had failed. The increasingly sectarian nature of the conflict is also fuelling international concern that the conflict could crack open the Middle East's religious and ethnic mosaic if unchecked.

Annan, a former Nobel Peace Prize laureate who is acting as Syria envoy for the UN and the Arab League, said he told Assad in "very direct and frank terms" when they met last Tuesday that the president must act to implement all points of the peace plan.

Annan said: "He must make bold and visible steps immediately to radically change his military posture and honour his commitment to withdraw heavy weapons and cease all violence.

"What is important is not the words he uses but the action he takes – now."

The massacre last week of more than 108 men, women and children in the eastern Houla region, believed by UN monitors to have been the work of pro-Assad militias and soldiers, caused renewed outrage. Many of the dead were women and children who were gunned down in their homes.

Since then, two other mass killings were reported, both on Thursday. Thirteen bound corpses, many apparently shot execution-style, were found in the eastern province of Deir el-Zour, near the Iraqi border. Gunmen also killed 11 people on their way to work at a state-owned fertiliser factory in the central province of Homs, activists said.

Annan said the Houla massacre was a terrible crime.

He added: "Worst of all, it is one of many atrocities to have taken place. Hundreds of thousands of Syrians are internally displaced. Meanwhile, arbitrary detentions continue, and alongside that, widespread allegations of human rights abuses of all kinds."

The UK Foreign Secretary, William Hague, said in Turkey: "Syria is on the edge. It is on the edge of a catastrophic situation, if we can imagine one even worse than the current situation."

Hague added: "We believe that the acts committed by the Syrian regime may amount to crimes against humanity and other international crimes, and demonstrate a pattern of widespread and systematic attacks against civilian populations."

He said evidence from UN observers and independent witnesses confirmed security forces shelled Houla and that "government militia then went house to house slaughtering entire families".

Satellite images released in the last few days show clearly the location of Syrian military forces around the site of last week's massacre. They also reveal what was thought to be the firing point for Syrian artillery that shelled villages where the massacre took place. While the images do not prove conclusively that the Syrian regime was responsible for the deaths on May 25-26, they do provide further detail of what is being called the single worst atrocity since the Syrian uprising began last year.

Forbes McKenzie, a former British Army intelligence officer who now works for the commercial company that analysed the images, said: "These images display a prominent footprint by the Syrian military in the locality of the massacres. We assess that it shows where 122mm artillery rounds were fired from on to the site of the massacre."

Other satellite images posted on the website of Robert Ford, the US Ambassador to Syria, showed signs of what look like freshly dug mass graves. A senior intelligence official confirmed the authenticity of the images.

Responding to the latest atrocities and asked if Assad should be allowed to leave power in exchange for safe haven, UN High Commissioner for Human Rights Navi Pillay said yesterday that international leaders seeking peace may be drawn to "politically expedient solutions which may involve amnesty or undertakings not to prosecute". But she said that would be wrong under international law.

Speaking in Brussels she said: "My message is very clear – there has to be accountability."

As the crisis deepens so, too, does the search for solutions with some Arab nations becoming more vocal.

Qatar's prime minister, Sheikh Hamid bin Jassim Al Thani, urged the UN to set a deadline for Annan's peace efforts and warn Assad that failure could mean invoking Chapter 7 of the UN charter, which allows for possible military action.

"We can't stand any more stalling," he said after an emergency meeting of Arab League foreign ministers to discuss Syria.

Arab League Secretary General Nabil Elaraby, meanwhile, suggested one option could be converting the Arab-led observer mission in Syria into a peacekeeping force.

A poll released yesterday showed a widening majority of French people in favour of military intervention. France controlled Syria between the 1920s and 1940s.

The Ifop poll showed backing for military intervention at 58%, up from 51% in February, and support for French involvement surging to 50% from 38%. Ifop said that the increase "is undoubtedly linked to the multiplication of war crimes blamed on Bashar al-Assad's regime and their recent media coverage".

France's new president, François Hollande, said last week that military intervention could not be ruled out as long as it was carried out under the auspices of a UN Security Council resolution.

Hollande is under pressure to show the same decisiveness and leadership that his conservative predecessor, Nicolas Sarkozy, did in the Libyan crisis last year.

Yesterday's flurry of diplomatic activity and responses from Annan, Pillay and the Arab League marked the starkest warning yet to the Assad regime that the levels of violence in Syria are totally unacceptable. Whether Assad will take heed is a very different matter.