The UN special envoy for Syria has vowed to take fragile peace talks into next week despite a walkout by the main opposition, a breakdown in a truce and signs that both sides are gearing up to escalate the five-year-old civil war.

The opposition declared a "pause" in the talks this week because of a surge in fighting and too little movement from the government side on freeing detainees or allowing in aid. Nearly all of its delegation left Geneva.

But UN envoy Staffan de Mistura said he had no plans to call off the negotiations, the first in five years of conflict to include some rebel factions. He said a ministerial meeting of world powers was urgently needed to get the talks back on track.

"Bottom line, I plan to continue the proximity talks, but at the formal level and at the technical level until next week, probably Wednesday as originally planned," he said.

A fragile ceasefire in place since February was still in effect because none of the sides had renounced it, he said, but it was "in great trouble if we don't act quickly".

The talks at UN 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.

De Mistura now says 400,000 people have been killed in the war, far higher than the previous UN toll, usually given as 250,000. He said he had no proof of the higher figure but the estimate of 250,000 was two years old and no longer valid.

The war was tilted in President Bashar al-Assad's favor late last year by Russia's intervention.

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 UN-led initiative to end the war or is using the talks to mask renewed military support for Assad.

President Barack Obama, on a visit to London, said the Syrian crisis cannot be solved without political negotiations and that required dealing with people he deeply disagrees with.

"We are not going to solve the overall problem unless we can get this political track moving," Obama told a joint news conference with British Prime Minister David Cameron.

He said he had always been sceptical about Putin's actions and motives in Syria and that Russia will recognise that the Syrian problem cannot be solved by military means.

Washington is leading its own campaign of air strikes against Islamic State positions in both Iraq and Syria. It acknowledged on Friday that 20 civilians were among those killed in its strikes between September 10 and February 2.

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

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."

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

A warplane crashed southeast of Damascus airport on Friday, said the Syrian Observatory for Human Rights, which monitors the conflict with a network of sources on the ground. It said the cause of the crash and the fate of the pilot were not clear.

The Islamic State group released a video claiming to have shot it down. Footage showed fighters around burning plane wreckage, part of which had a Syrian flag painted on it. Reuters could not independently verify the video.

Russia's Interfax news agency quoted a Syrian military source saying it crashed due to technical malfunction.

In Aleppo, government air strikes in different parts of the city killed at least 19 people and wounded dozens more on Friday, with the death toll expected to rise due to serious injuries, the Observatory 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 Observatory.

Assad main issue

Endorsed by the UN 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 towards 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.

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.