The spark was lit in March 2011, when a group of young boys were arrested and cruelly tortured for daubing walls in the southern Syrian city of Daraa with anti-government graffiti.

A year and a half on, a once peaceful uprising against President Bashar al-Assad inspired by the Arab Spring revolts against other autocratic regimes has descended into brutal civil war with no endgame in sight, analysts say.

Men, women and children are trapped in bombed-out towns, people are struggling to find food or medical supplies to treat the sick and wounded, while the grim cycle of shelling and air strikes and fighting claims scores of lives daily.

"There are a lot of people who say that we must avoid civil war in Syria. I believe that we are already there for some time now. What's necessary is to stop the civil war and that is not going to be easy," new peace envoy Lakhdar Brahimi said on Sunday, describing it as the "cruellest kind of conflict."

The International Committee of the Red Cross began treating the conflict as a civil war more than a month ago, when the fighting dramatically escalated, reaching the capital Damascus and the main northern city of Aleppo.

But the foreign ministry angrily dismissed the term.

"To speak of civil war in Syria contradicts reality and is found only in the head of conspirators," it said.

Nevertheless the death toll has risen alarmingly in the face of international deadlock. It has surged from 2,200 in August 2011 to more than 23,000 now, according to the Syrian Observatory for Human Rights, while the UN puts the figure at around 17,000.

As the conflict grinds bloodily into its 18th month, the regime is resorting to even heavier firepower against the rebels, a motley collection of fighters, military defectors and Islamists groups loosely united as the Free Syrian Army.

They are concentrated in the northwestern provinces of Aleppo and Idlib and Deir Ezzor in the far east, and parts of the central Homs province.

Activists are reporting increasing attacks from the air by regime forces, alongside relentless daily shelling and sniper attacks in the rebel strongholds.

The government warned last month that Aleppo should brace for the "mother of all battles" but has failed to dislodge opposition fighters from many districts of Syria's second city and is battling pockets of resistance even in the capital and other parts of the country.

"The level of violence being exerted by the state indicates that they are on the cusp of losing control," a top Western military officer with experience in Afghanistan and Iraq told AFP.

-- Defections --

----------------

The regime has already been dealt major body blows with the defection of former prime minister Riad Hijab and high-profile general Manaf Tlass, while a string of generals has fled to Turkey to join the rebellion.

Assad also lost his defence minister and three other top security officials in a bomb attack last month claimed by the FSA.

Hijab said after fleeing to Jordan the regime controls only 30 percent of Syria and had "collapsed militarily, economically and morally," while there is talk of further high-profile defections to come.

"The regime cannot 'win,' nor can it survive indefinitely," Yezid Sayigh, a senior associate at the Carnegie Middle East Centre, said in a commentary.

Assad, from the minority Alawite sect, insists he is fighting for the very future of Syria in the face of a foreign "terrorist" plot aided by the West and regional rivals, including Sunni powerhouse Saudi Arabia.

In a sign of the increasingly sectarian nature of the conflict, activists say there has been a spate of tit-for-tat kidnappings and cold-blooded executions by rival Sunnis and Alawites.

Analysts say the "tipping point" for Assad -- who defiantly refuses Western calls to go -- will be when there are large-scale defections by the government and the armed forces, but that for now the top echelon is probably intact if deeply shaken.

"Vestiges of the state may continue to function in Syria and the current regime may cling to power, but there is no mistaking that Syria is ensnared in a civil war," Augustus Richard Norton, a Middle East expert at Boston University, told AFP.

As the conflict becomes more vicious by the day, a UN panel said last week the regime has committed crimes against humanity, including the Houla massacre that shocked the world, but also said rebels were guilty of war crimes, although to a lesser extent.

"It is clear that both sides have chosen the path of war," said Edmond Muret, a UN peacekeeping chief as he announced Sunday's end of the observer mission.

Analysts warn the conflict could become radicalised and even entangle Syria's neighbours, particularly fragile Lebanon, and other countries sheltering tens of thousands of refugees.

UN chief Ban Ki-moon has described it as a "proxy war" being played out between regional and international rivals.

The West and its Sunni Muslim allies back the rebels, while Shiite Iran remains Syria's staunchest supporter and Russia and China steadfastly block UN resolutions on the conflict, rejecting any foreign intervention.

The International Crisis Group's Peter Harling warned this month of "ominous trends" as each side becomes more entrenched -- the regime and its Alawite supporters in a "desperate fight for survival" while the opposition is threatened from within by "sectarianism, retaliatory violence and fundamentalism."

"Together, this could portend a prolonged, ever more polarised, destructive civil war," he said.