Major power foreign ministers failed to agree a new date to resume Syrian peace talks at a meeting on Tuesday, and the opposition said it would not come back to Geneva negotiations unless conditions improved on the ground.

A pessimistic atmosphere pervaded the meeting in Vienna between countries that support President Bashar al-Assad and his enemies, all of which have committed to reviving a ceasefire and peace process that have been unravelling since last month.

In a joint statement after the meeting attended by the United States, European and Middle East powers that oppose Assad as well as Russia and Iran which support him, the powers called for a full cessation of hostilities and access for aid.

In stronger language than in the past, they warned the warring factions that if they repeatedly broke the truce they risked forgoing the protection of the Feb. 27 cessation of hostilities agreement sponsored by Washington and Moscow.

They also directed the U.N.'s World Food Programme to air drop food, medicine and water to besieged communities starting on June 1 if humanitarian access was denied by either side.

But they did not agree on a date for peace talks to resume. The Geneva talks broke up last month after the opposition delegation quit, accusing the government of ignoring the ceasefire, and recent weeks saw an intensification of fighting, particularly near Aleppo, Syria's largest city before the war.

U.N. envoy Staffan de Mistura told a news conference there was still a strong desire to keep the peace process moving.

"We want to keep the momentum. The exact date, I am not at the moment revealing it because it will depend also on other facts," he said. He noted that the Muslim fasting month of Ramadan, which begins in early June, was coming soon.

The main opposition High Negotiations Committee said it was not willing to return to negotiations without a full ceasefire and access for humanitarian aid.

"I don't think there will be results, and if there are any results they will not be sufficient for the Syrian people," HNC chief negotiator Asaad al-Zoubi told Reuters ahead of Tuesday's Vienna meeting. "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."

After Tuesday's meetings, the opposition said the powers had rebuked Assad by demanding that aid should reach towns his forces were blockading.

"The Vienna Communiqué listed towns where our fellow Syrians are starving under siege and crying out for humanitarian access," HNC spokesman Salem al-Meslet said in a statement. "Assad is not only blockading those towns, but also blocking the path to a political solution, which is the only way to end the suffering."

But HNC member Bassama Kodmani told Reuters the powers must do more to silence the guns: "We cannot be bombed while we're talking about a peaceful arrangement and a peaceful transition."

Local truces in individual areas, which have been attempted in recent weeks, would not be a solution. "Peace talks cannot take place while one front is burning and another is quiet," she said.

NO PEACE WITHOUT SETTLEMENT

Washington, which wants Assad to leave power, has worked closely on diplomacy with Russia, which joined the war last year to support him. That has alarmed some of Assad's opponents. Some diplomats and former officials say Washington may have underestimated Moscow's desire to keep Assad in power.

U.S. Secretary of State John Kerry, standing beside Russian Foreign Minister Sergei Lavrov and de Mistura at Tuesday's news conference, made clear Washington still wanted Assad out.

Russia needed to use its influence over Assad to secure a transition in Syria, Kerry said.

"This war will not end for him or for his people without a political settlement."

Gesturing to Lavrov, Kerry said Assad had made a series of commitments to Moscow that he was prepared to negotiate, but had not kept his word.

"I think he should never make a miscalculation about President Obama's determination to do what is right at any given moment of time where he believes he has to make that decision," Kerry said.

Lavrov repeated Moscow's line that it was not fighting on behalf of any particular ruler in Damascus: "We don't support Assad. We support the fight against terrorism."

French Foreign Minister Jean-Marc Ayrault said progress was needed urgently on the ground.

"If nothing happens in terms of respecting the ceasefire or humanitarian aid, then it will no longer be about discouragement, but despair. We are in an extremely fragile period."

AID, VIOLENCE

The United Nations said this month that Syria's government, which has been on the front foot in the war since its Russia intervened last year, was refusing U.N. demands to deliver aid to hundreds of thousands of people.

Tuesday's talks discussed ways to stop the violence by separating al-Nusra Front, al Qaeda's wing in Syria, from other opposition fighters in some areas like Aleppo.

Ayrault said France had told moderate opposition groups "they must be extremely clear with regard to groups like al-Nusra. There must be no ambiguity".

Nusra, along with Islamic State, is not a party to the ceasefire. Western and Arab states accuse the Syrian government and Russia of using links between rebels and Nusra as a pretext to launch offensives against other opponents of Assad.

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 U.N. Security Council resolution. Kerry said in Vienna that if progress in talks was made, the timeframe would be respected.

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 five-year civil war, diplomats say.

Saudi Foreign Minister Adel al-Jubeir said it was perhaps time to consider alternatives, including ramping up military aid to rebels, if Assad's government continued to flout international agreements.

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 the besieged town of Daraya on the outskirts of Damascus said they believed government forces were preparing an assault.

Last week government forces refused entry to what would have been the first aid convoy to reach the town. Troops began shelling the town on Thursday, ending a lull that had prevailed since the ceasefire took effect. Residents say they are on the verge of starvation.

"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 blocked aid convoy 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". – Reuters