MUNICH (Reuters) - Major powers began a new round of Syria talks on Thursday focussing on calls for a ceasefire and access for aid, but the mood was dour with Moscow showing no sign of calling off its bombing in support of a massive new government advance.

Russian Prime Minister Dmitry Medvedev raised the spectre of an interminable conflict or even a world war if powers failed to negotiate an end to the fighting in Syria, which has killed 250,000 people, caused a refugee crisis and empowered Islamic State militants.

With the Syrian opposition saying it cannot accept a truce because it does not trust the Russians, diplomats saw little chance of progress at the meeting in the German city of Munich.

The first peace talks in two years between belligerents in Syria collapsed last week before they began in the face of the offensive by President Bashar al-Assad’s forces, one of the biggest and most consequential of the five-year war.

Thursday’s meeting was meant to allow powers to coordinate support for ongoing negotiations, but instead has turned into a desperate bid to resurrect them.

Ministers wrangled over three core issues: a gradual cessation of hostilities with a firm end date, humanitarian access to cities being besieged by both sides and a commitment that Syrian parties return to Geneva for political negotiations.

U.S. Secretary of State John Kerry has repeatedly urged Moscow since peace talks broke down in Geneva to halt its bombardments in Syria in support of Assad.

Moscow, however, had proposed a truce that would begin only from the start of next month, giving its Damascus allies 18 more days to recapture Aleppo, once Syria’s largest city.

“Here we need something of a breakthrough,” said German Foreign Minister Frank-Walter Steinmeier. “Today, we will try what has not been achieved so far especially, to get better supplies to people locked in Syria and link this to first steps in a significant reduction of violence.”

But a senior Western diplomat gave a pessimistic outlook: “This meeting risks being endless and I fear the results will be extremely small.”

Russia’s intervention on the battlefield since last year has swung the momentum. Government forces and allies have routed rebels and come close to encircling Aleppo, a divided city half held by rebels for years.

Russia’s Foreign Minister Sergei Lavrov said Moscow had submitted proposals for a ceasefire and was awaiting a response from other powers. But Western officials do not expect Moscow to accept the immediate halt to bombing that Washington seeks.

Kerry said he expected a “serious conversation”.

“Obviously, at some point in time, we want to make progress on the issues of humanitarian access and ceasefire,” Kerry said.

Riad Hijab, chief coordinator of the main Western-backed Syrian opposition, told reporters he hoped to see progress quickly, including a “working group” to press ahead on the core issues.

He warned that there was little trust of Russia among the opposition.

“What Russia, the regime, Iran and the sectarian militias supported by Iran, do doesn’t serve the peace process, quite the opposite, it hinders the peace process,” he said.

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Russia is widely viewed as unlikely to halt support for the government advance until Damascus achieves its two main objectives: recapturing Aleppo and sealing the Turkish border, for years the lifeline for rebel-held areas.

That would amount to its most decisive victory of the war so far, and probably put an end to rebel hopes of removing Assad by force, their goal throughout years of fighting that has driven 11 million people from their homes.

“The goal is to totally liberate Aleppo and then to seal the northern border with Turkey,” said Ivan Konovalov, director of the Center for Strategic Trend Studies in Moscow, explaining the Russian government thinking. “The offensive should not be stopped - that would be tantamount to defeat.”

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TURKISH BORDER

Turkey has already taken in 2.6 million Syrians, the world’s largest refugee population, and has agreed to help keep them from travelling into Europe in return for aid. Erdogan warned that Turkey could “open the gates” for refugees into Europe if it did not receive enough help.

The United Nations and the European Union, which has agreed a 3 billion euro fund to improve conditions for refugees in Turkey, have both urged Ankara to admit those fleeing the fighting.

“They struck Aleppo so we fled. First we escaped to another village. We’ve gone to every village. But they’re bombing everywhere so we came here,” said Musa Ibrahim Isa, one of the tens of thousands of people at Bab al-Salama, on the Syrian side of the Turkish border.

“Our only wish from God is that these gates be opened.”

President Tayyip Erdogan said in a speech that Turkey’s patience may run out and Ankara may have to take action, but gave no details of what he meant. Erdogan called on the United Nations to prevent “ethnic cleansing”, saying as many as 600,000 more refugees could arrive.

The NATO alliance announced a new sea mission to help Turkey and Greece crack down on criminal networks smuggling refugees into Europe from Turkey, after thousands drowned and hundreds of thousands made the journey last year.

Washington is leading its own air campaign against Islamic State militants in eastern Syria and northern Iraq, but has resisted calls to intervene in the main battlefields of Syria’s civil war in the west of the country, where the government is mostly fighting against other insurgent groups.

That has left the field to the Russians, who support Assad against an array of rebel groups backed by Turkey, Arab states and the West.

The United States pressed allies to contribute more to the U.S.-led military campaign against Islamic State, which it said must be accelerated regardless of the fate of diplomatic efforts to end Syria’s civil war.