By John Davison and Stephanie Nebehay

BEIRUT/GENEVA (Reuters) – The U.N. special envoy for Syria has vowed to take fragile peace talks into next week despite a walkout by the main armed opposition, a breakdown in a truce and signs that both sides are gearing up to escalate the five-year-old civil war.

Staffan de Mistura, who dismissed the opposition’s departure as “diplomatic posturing”, expected the delegation to return to the negotiating table. The opposition declared a “pause” this week because of a surge in fighting and too little movement from the government side on freeing detainees or allowing in aid.

Asked whether talks would carry on, de Mistura said on Thursday night: “We cannot let this drop. We have to renew the ceasefire, we have to accelerate humanitarian aid and we are going to ask the countries which are the co-sponsors to meet.”

The talks at U.N. headquarters in Geneva aim to halt a conflict that has allowed for the rise of the Islamic State group, sucked in regional and major powers and created the world’s worst refugee crisis.

In an interview with French-language Radio Television Suisse (RTS), de Mistura said 400,000 people had been killed in the war, far higher than the previous U.N. toll which has varied from 250,000 to 300,000.

The war was tilted in Assad’s favor late last year by Russia’s intervention, supported on the ground by members of the Iranian Revolutionary Guard Corps who have been bolstered recently by the arrival of members of Iran’s regular army.

WASHINGTON CONCERNED BY RUSSIAN MOVES

The White House has expressed concern that Russia has repositioned artillery near the disputed city of Aleppo.

The Russian military moves have sharpened divisions in Washington over whether President Vladimir Putin genuinely backs the U.N.-led initiative to end the war or is using the talks to mask renewed military support for Assad.

“The regime is so reliant on external support that it is inconceivable that its allies don’t have the leverage to change its approach,” Britain’s envoy to the Syria peace talks, Gareth Bayley, said on Friday.

Russian Foreign Minister Sergei Lavrov said on Friday that the decision by the opposition High Negotiations Committee (HNC) to quit Geneva was not a loss for anyone except the HNC itself.

“If they want to ensure their participation (in the peace talks) only by putting ultimatums, with which others must agree, it’s their problem,” Lavrov said, adding:

“For God’s sake, we shouldn’t be running after them, we must work with those who think not about their career, not about how to please their sponsors abroad, but with those who are ready to think about the destiny of their country.”

The head of the Syrian delegation, Bashar Ja’afari, confirmed he met de Mistura to discuss humanitarian issues on Friday and would be meeting with him again on Monday.

Moscow and Washington sponsored the fragile cessation of hostilities that went into effect on Feb. 27 to allow talks to take place but has been left in tatters by increased fighting in the past week.

In Aleppo, government air strikes in different parts of the city killed at least 10 people and wounded dozens more on Friday, with the death toll expected to rise due to serious injuries, the Syrian Observatory for Human Rights said.

Further southwest in Hama province, warplanes targeted rebel-held areas in the strategic Ghab plain that borders Latakia province, Assad’s coastal heartland.

Insurgents announced a new battle in Latakia earlier this week which they said was in response to ceasefire violations by the government side, launching fierce assaults there. Fighting raged in the area on Friday, said the British-based Observatory, which monitors the conflict with a network of sources on the ground.

ASSAD MAIN ISSUE

Endorsed by the U.N. Security Council, the Geneva peace talks marked the most serious effort yet to resolve the war, but failed to make progress on political issues, with no sign of compromise over the question of Assad’s future.

Government negotiators say Assad’s presidency is non-negotiable. Underlining confidence in Damascus, a top Assad aide reiterated its view that local truce agreements and “destroying terrorism” were the way toward a political solution.

The opposition wants a political transition without Assad, and says the government has failed to make goodwill measures such as releasing detainees and allowing enough aid into opposition-held areas besieged by the military.

The HNC, which is backed by Western nations and key Arab states, had this week urged more military support for rebels after declaring the truce was over and said talks would not re-start until the government stopped committing “massacres”.

All the main HNC members had left Geneva by Friday, leaving a handful of experts and a point of contact behind.

With violence escalating, peace talks might not resume for at least a year if they are abandoned, one senior Western diplomat said.

Syria is now a patchwork of areas controlled by the government, an array of rebel groups, Islamic State, and the well-organized Kurdish YPG militia.

On Friday, rare clashes between YPG fighters and government-allied forces and militiamen took place for nearly a third day, the Observatory said, in fighting which a Syrian Kurdish official said had killed 26 combatants. Kurdish and government forces have mostly avoided confrontation in the past.

(Additional reporting by John Irish in Geneva; writing by Peter Millership; editing by Peter Graff)