World and regional powers have agreed to try and turn a fragile truce in Syria into a comprehensive ceasefire.
Meeting in Vienna, the global powers have also set a deadline of June 1 for the resumption of humanitarian aid to areas cut off from the outside world.
US secretary of state John Kerry said that if land routes remain blocked, food aid will be air dropped and international pressure will be increased on those preventing such relief from getting through.
He added that pressure will also be applied to stop indiscriminate use of force by the Syrian military.
But the meeting failed to devise any concrete ways of resolving the factional divisions driving the five-year war that has left hundreds of thousands dead and fuelled the rise of Islamic extremists.
Mr Kerry told reporters “a variety of competing interests are going to have to be reconciled” in order to end the conflict.
“Those involved in this conflict with competing agendas are going to have to prioritise peace,” he said.
German foreign minister Frank-Walter Steinmeier spoke of “difficult, in part controversial” talks, which he described as normal “when 20 nations with very different experiences and a different view of Syria sit at one table”.
One key division continues to be the fate of Syrian President Bashar Assad. Moscow opposes any attempt to forge a peace settlement that is conditional on his removal. But Russian foreign minister Sergey Lavrov said his country’s support of the Syrian government did not constitute backing for Assad.
Instead, he said Russia supports “the fight against terrorism, and we don’t see a better alternative to doing that than the Syrian army”.
Going into the talks, Mr Steinmeier repeated the position held by the West and the Saudi-backed opposition that an agreement should outline steps leading to the end of the Syrian leader’s rule.
He told reporters: “This is necessary because there can be no lasting future for this country with Assad. This is why we must start negotiations here in Vienna … about what a transition government could look like.”
Mr Kerry said that “without a negotiated solution, Assad and his supporters will never end the war”.
And he questioned suggestions that Assad was immune from international pressure to agree to a settlement, implying that other means could be applied if the Syrian leader refuses to co-operate.
If Assad “has reached a conclusion that there is no Plan B, he has done so without any foundation whatsoever, and it’s very dangerous, dangerous,” Mr Kerry said.
In a nod to Moscow, Assad’s key international backer, Mr Kerry said Russia “has made it very clear” that Assad has signed on to commitments that include participation in peace talks, constitutional change and elections.
“But he has yet to live up to the first one, which is to participate fully in the Geneva talks on a political transition,” he said.
Mr Kerry said the diplomats at the talks also called on all parties to dissociate themselves from Islamic State and the al Qaida affiliate, known as the Nusra Front.
Those comments reflect international concerns about attempts by Islamic radicals to form alliances with Syrian rebels, a concern Mr Lavrov said all participants at the talks share.
“In particular, we have the problem of al-Nusra,” he said. “It is changing, it makes alliances with groups in the cessation of hostilities.”
The talks, which included foreign ministers or other senior officials from more than 20 countries and organisations, were convened after discussions meant to reduce differences between rival factions sputtered last month in Geneva as fighting flared.
The current effort to end the five-year Syria conflict was largely spearheaded by Mr Kerry and Mr Lavrov, backed by major global and regional powers that formed the International Syria Support Group.
A truce brokered by the US and Russia sharply reduced violence in March, but that truce has been steadily eroding. The Vienna conference was called after UN Syria envoy Staffan de Mistura appealed last month to Washington and Moscow to directly intervene in putting the Syria dialogue back on track.
The Geneva talks stalled after the Western and Saudi-backed opposition suspended formal participation in the indirect talks with Assad’s envoys to protest alleged government ceasefire violations, a drop in humanitarian aid deliveries and no progress in winning the release of detainees in Syria.