By John Irish and Suleiman Al-Khalidi
VIENNA/AMMAN (Reuters) - Major powers sought at talks on Tuesday to reimpose a ceasefire in Syria and ensure aid reaches besieged areas, with Moscow and Washington deeply divided over the fate of President Bashar al-Assad and violence rumbling around the country.
The aim of the conference, which brings together 17 countries backing the two warring sides, is to convince armed factions and opposition leaders to restart negotiations with the government.
Officials and diplomats said the talks, including the United States, Russia, Iran, European and Middle East powers, were unlikely to lead to major decisions that could change the course of the five-year war that has killed more than 250,000 people.
A surge in bloodshed in Aleppo, Syria's largest city before the war, wrecked a partial "cessation of hostilities" sponsored by Washington and Moscow from February, which had allowed U.N.-brokered indirect talks that included the warring sides to take place in Geneva.
Those talks collapsed last month after the opposition walked out due to a surge in bloodshed. U.N. special envoy for Syria Staffan de Mistura hopes to launch a new round of peace talks between the two sides by the end of May.
"We'll need to see the guarantors of the ceasefire - Russia and the U.S. - putting something down that will really convince the opposition that this process is worthwhile," a senior Western diplomat involved in the talks said.
"Sadly, I don't sense that and fear the U.S. will try to impose a text that is excessively optimistic, but for which its implementation will not be possible."
The Geneva talks aim to end a war that has created the world's worst refugee crisis, allowed for the rise of the Islamic State group and drawn in regional and global powers.
Washington insists Assad must go but the president, backed by Moscow and Tehran, is fighting for territory and refuses to step down.
AID AND LOGISTICS
A Western official said the meeting, chaired by U.S. Secretary of State John Kerry and Russian Foreign Minister Sergei Lavrov, was focusing more on the logistics of expanding and implementing the "cessation of hostilities" and increasing aid deliveries that have been blocked in some areas.
The United Nations said this month that Syria's government, which has been on the front foot in the war since the military intervention of its ally Russia, was refusing U.N. demands to deliver aid to hundreds of thousands of people.
Describing the talks as serious and engaged, another Western diplomat said one of the key issues was stopping the violence in a way which successfully separated al-Nusra Front, al Qaeda's wing in Syria, from opposition fighters.
Western and Arab states accuse the Syrian government and Russia of using links between rebels and Nusra as a pretext to launch major offensives against Western and Arab-backed opponents of Assad. Nusra, along with Islamic State, is not party to the ceasefire.
"We must find a way back into the political process ... It's about improving the conditions for the ceasefire and humanitarian aid so as to win the opposition over to negotiate with the regime in Geneva," German Foreign Minister Frank-Walter Steinmeier said ahead of the meeting.
The main Syrian opposition's High Negotiations Committee has said it would not resume talks until there was progress on the ground.
HNC chief negotiator Asaad al-Zoubi was doubtful about how much the Vienna talks can achieve: "I don't think there will be results, and if there are any results they will not be sufficient for the Syrian people," he told Reuters.
"We are used to the fact that Russian and U.S. foreign ministers are taking the world into an unknown direction," saying they are working against the people not for them.
The group walked out of the Geneva talks. Asked if the HNC would return to another round of talks, al-Zoubi said: "The HNC has said that if aid does not reach everybody, if the sieges aren't lifted and if a full truce does not happen, there will be no negotiations."
De Mistura is trying to meet an Aug. 1 deadline to establish a transitional authority for the country that would lead to elections in 18 months as agreed in a December United Nations Security Council resolution.
FATE OF ASSAD
However, the U.S. administration's failure to convince Moscow that Assad must go is fuelling European and Arab frustration at being sidelined in efforts to end the country's five-year civil war, diplomats say.
Some diplomats and analysts question whether the United States has misread Russia's desire to keep Assad in power.
In the past weeks, several hundred civilians have been killed in air strikes and rebel bombardments in Aleppo province alone, while fighting has taken place in other parts of Syria, including Idlib, Deir al-Zor and around Damascus.
As the talks took place, rebel fighters and officials in a besieged Syrian town on the outskirts of Damascus said they believed government forces were preparing an assault after they turned back an aid convoy last week.
Daraya, situated close to a large air base and just a few kilometers (miles) from Assad's palace, had seen little violence since the cessation of hostilities agreement came into effect.
But, with the truce unraveling across Syria, government forces began shelling the town on Thursday after refusing entry to the first aid convoy it would have ever received. Residents say they are on the verge of starvation.
Known for its peaceful protests in the early days of the uprising against Assad, Daraya has been besieged and regularly bombed since 2012.
"Large convoys of (government) troops are moving from the airport and from Ashrafiyat Sahnaya (the next town south)," said Abu Samer, spokesman for the Liwa Shuhada al-Islam rebel group.
"We are prepared to repel their assault but our main fear is for the civilians besieged in the town who face severe shortages of food."
A Syrian military source denied rebel accounts of troop deployments, saying nothing had changed in the area.
The aid convoy blocked last week would have been the first delivered since the siege began. But even then it was not allowed to contain food, only medical and other aid, and residents launched an online campaign ahead of the expected delivery with the slogan: "We cannot eat medicine".
(Additional reporting by Lesley Wroughton and Shadia Nasralla in Vienna and Lisa Barrington in Beirut; writing by Peter Millership; editing by Peter Graff)