Syria peace talks due to begin in Geneva this week look set to struggle, with the sides showing no sign of compromise over the issue at the heart of the five-year-long conflict: the future of President Bashar al-Assad.

The U.N.-led talks getting under way on Monday with US and Russian support are part of the first serious diplomatic effort towards ending the conflict since Moscow intervened last September with air strikes that have tipped the war Assad's way.

With the crisis approaching its fifth anniversary this week, Western states seem more determined to bring an end to a war that has driven hundreds of thousands of refugees towards Europe and helped the rise of Islamic State.

But while recent cooperation between the United States and Russia has helped to reduce the level of violence and brought the parties to Geneva, the positions of the government and opposition reveal little ground for a negotiated settlement.

Pointing to a possible escalation in the war if there is no progress, the Russian defence ministry said rebels had used an anti-aircraft missile to shoot down a Syrian warplane on Saturday.

Rebels said it was shot down with anti-aircraft guns, rather than a missile, a weapon fighters have sought but Western countries want to keep out of their hands because of the potential threat to civil aviation if Islamist militants acquire them.

Reflecting the Damascus government's confidence, Foreign Minister Walid al-Moualem warned the opposition on Saturday it was deluded if it believed it would be able to take power at the negotiating table, and ruled out any talks on the presidency.

READY TO FIGHT ON

The opposition are holding out little hope that Geneva will bring them nearer to their goal of toppling Assad. Announcing its decision to attend the Geneva talks, the main opposition umbrella group said the government was preparing for more war.

Rebels say they are ready to fight on despite their recent defeats. They hope foreign backers will send them more powerful weapons including anti-aircraft missiles if the political process collapses.

"I expect that if in this round the regime is stubborn, and doesn't offer anything real, it will be the end of the talks and we will go back to the military solution," said Bashar al-Zoubi, a prominent rebel.

The talks aim to build on a "cessation of hostilities" agreement brokered by the United States and Russia that has brought about a considerable reduction in fighting since it came into effect on Feb. 27.

It marks the most serious effort yet towards de-escalating the conflict, surprising many and allowing for aid deliveries to besieged areas, though the opposition says the deliveries to rebel-held territory fall well short of needs.

The sides have, however, accused each other of violations, and Saturday was one of the most violent days since it came into force, with rebels and government forces clashing in Hama province and insurgents shooting down the warplane.

The Russian defence ministry said a portable air-defence system had been used to bring down the Syrian MiG-21.

"Russia wants to accuse the friends of the Syrian people of supplying it with missiles, and this did not happen," said Mohamad Alloush, head of the politburo of the Jaish al-Islam group and HNC chief negotiator.

He said all groups were requesting the means to defend civilians from warplanes and barrel bombs - oil drums filled with explosives that the opposition says the army uses to cause indiscriminate damage in rebel areas.

REBELS UNDER PRESSURE

The main opposition alliance, known as the High Negotiations Committee (HNC), comes to the talks with the balance of forces stacked against it after Russia's intervention and an increase in military support to Assad from Iran, his other main ally.

The HNC has also voiced concerns about what it sees as a softening of the US stance on Syria, saying Washington has given ground to Moscow. HNC official George Sabra, speaking in Geneva, called the American position "ambiguous, even for its allies".

The HNC says the talks must focus on setting up a transitional governing body with full executive powers, and that Assad must leave power at the start of the transition.

But Foreign Minister Moualem on Saturday set out a very different vision, indicating that the most the government would offer was a national unity government with opposition participation, and a new or amended constitution.

He also said the government delegation would resist any attempt to put the question of presidential elections on the agenda, and criticised U.N. envoy Staffan de Mistura for last week outlining an agenda that includes elections.

De Mistura is due meet the sides separately on Monday before briefing the Security Council.

"SPOILER"

Syria's U.N. ambassador Bashar Ja'afari, head of the government delegation, said the talks needed to work on preparatory issues first and it was premature to talk about a transitional period.

US Secretary of State John Kerry said Moualem's comments aimed to disrupt the political process.

Kerry also said the Syrian government and its backers were mistaken if they thought they could continue to test the boundaries of the fragile truce. Accusing Damascus of carrying out the most violations, Kerry said Russian President Vladimir Putin needed to look at how Assad was acting.

"President Assad is singing on a completely different song sheet and sent his foreign minister out yesterday to try to act as a spoiler and take off the table what President Putin and the Iranians have agreed to," Kerry said.

Attempts to get the diplomatic process moving have already faced big obstacles, including a row over who should be invited to negotiate with the Syrian government. The HNC groups political and armed opponents of Assad.

Russia reiterated its view that the Kurdish PYD party, which wields wide influence in northern Syria, should be at the talks. The PYD has been excluded in line with the wishes of Turkey, which views it as an extension of the PKK group that is waging an insurgency in southeastern Turkey.

Russian Foreign Minister Sergei Lavrov said Moscow had evidence that Turkish armed forces were on Syrian territory, calling Turkey's actions "creeping expansion". There was no immediate Turkish response, but Ankara has in the past repeatedly denied that it was planning an incursion.

Though not invited, PYD leader Saleh Muslim told Reuters he hoped the talks would not fail, adding: "If they do, the results will be disastrous for everyone." – Reuters