World powers have agreed to try to turn the crumbling partial truce in Syria into a more comprehensive ceasefire.
The International Syria Support Group (ISSG) warned that the unravelling of the 11-week cessation of hostilities could lead to a return to all-out war.
Those persistently breaching the truce could be excluded from it.
The ISSG also said that from June the UN would begin air drops of aid for all areas in need if ground access to besieged areas continued to be denied.
Diplomats want to encourage the opposition to resume indirect negotiations on a political settlement to end the five-year conflict, which has killed more than 250,000 people.
Opposition representatives suspended their participation in the last round in April in protest at mounting violence and stalled aid deliveries.
In the latest fighting, 50 people were reportedly killed in clashes between rival rebel factions outside Damascus.
The Syrian Observatory for Human Rights said the fighting in the Eastern Ghouta region involved members of Jaish al-Islam, which is part of the main which is part of the opposition High Negotiations Committee (HNC), and Faylaq al-Rahman, backed by jihadist militants from al-Nusra Front.
More than 500 people had now been killed since the rebel infighting erupted in late April, the UK-based monitoring group added.
Tuesday's meeting in Vienna involved 21 countries and organisations in the ISSG, many of which have intervened directly or indirectly in Syria's five-year civil war.
They include the US, UK, France, Germany, Saudi Arabia and Turkey, which support the opposition, along with Russia and Iran, which back President Bashar al-Assad's government.
The ISSG has set a deadline of 1 August for the formation of a transitional government in Syria, leading to elections within 18 months.
However, government negotiators have shown no sign of engaging in substantive talks on the matter, with the future of President Assad the main sticking point.
US Secretary of State John Kerry said: "A variety of competing interests are going to have to be reconciled.
"Those involved in this conflict with competing agendas are going to have to prioritise peace."
Mr Kerry called on all sides to intensify efforts to stop all indiscriminate use of force, stop bombing civilian areas, and disavow any link with the jihadist groups, Islamic State (IS) and al-Nusra Front, which are excluded from the cessation of hostilities.
Russia's Foreign Minister Sergei Lavrov also stressed that all sides had to seek a compromise in the political aspect of the peace talks.
The UN special envoy for Syria, Staffan de Mistura, reportedly hopes to launch a new round of negotiations in Geneva by the end of May.
But the Italian-Swedish diplomat said he could not issue invitations until a "credible" ceasefire was in place.
Meanwhile, Saudi Arabia's foreign minister said the opposition's allies needed to consider a "Plan B" if the talks failed.
"The choice about intensifying the military support is entirely with the Bashar regime. If they do not respond to the treaties of the international community then we will have to see what else can be done," Adel al-Jubeir warned.
In April, US media reports suggested Washington was ready to provide rebel forces with much more sophisticated weapons to strengthen their hand.