The United Nations envoy for Syria appealed to the U.S. and Russia to intervene to help revive Syrian peace talks that sputtered to a pause Wednesday, saying a recent spike in fighting has overshadowed the talks and put an increasingly feeble truce in "great danger."

Staffan de Mistura spoke to reporters early Thursday after briefing the UN Security Council via videoconference about the largely stalled indirect talks between the Western- and Saudi-backed opposition and envoys from President Bashar al-Assad's government, which has the backing of Moscow.

After 60 days, he said, the cessation of hostilities agreed to by both sides "hangs by a thread."

A boy, who was among those forced to flee from two rebel-besieged towns of Foua and Kefraya, is seen standing inside a hospital that was struck by an explosion on the outskirts of the Sayeda Zeinab district south of Damascus, Syria, on Monday. A surge of violence in Syria has put peace talks in danger of unravelling, says the UN's envoy to the country. (Omar Sanadiki/Reuters)

"I really fear that the erosion of the cessation is unraveling the fragile consensus around a political solution, carefully built over the last year," de Mistura said in his council briefing obtained by The Associated Press. "Now I see parties reverting to the language of a military solution or military option. We must ensure that they do not see that as a solution or an option."

The current effort to end the five-year Syria conflict was largely spearheaded by U.S. Secretary of State John Kerry and Russian Foreign Minister Sergey Lavrov, backed by major global and regional powers who formed the International Syria Support Group. It includes 17 countries as well as the United Nations, Arab League, European Union and Organization of Islamic Cooperation.

'Downward spiral'

De Mistura appealed not only for U.S.-Russia intervention but for international support, calling in the Security Council briefing for another ministerial meeting of the International Syria Support Group "lest we lose the window of opportunity to reverse the negative downward spiral."

He stressed that the talks may not survive unless the fighting subsides, and that won't happen "without some sort of political solution on the horizon."

How can you have substantial talks when you have only news about bombing and shelling? - Staffan de Mistura, UN envoy to Syria

De Mistura said he hoped that the talks would resume in May, and he predicted the overall process would continue as previously planned through July. But he stopped short of setting a specific date, pointing to the recent upsurge in fighting, notably in and around Aleppo, Syria's largest city before the war.

Lamenting the worsening violence, he said that "in the last 48 hours, we have had an average of one Syrian killed every 25 minutes, one Syrian wounded every 13 minutes ... How can you have substantial talks when you have only news about bombing and shelling?"

Speaking at the end of a third session of Syria peace talks this year, de Mistura said the truce brokered by the U.S. and Russia had pulled off a "miracle" by sharply reducing violence in March, but acknowledged that the renewed fighting has put the cessation of hostilities "in great danger."

U.S., Russia must work together: UN

He called for a "U.S.-Russian initiative at the highest level" to help reinforce it.

"There is no reason that both of them — who have been putting so much political capital in that success story and have a common interest in not seeing Syria ending up in another cycle of war — should not be able to revitalize what they created, and which is still alive, but barely," he said of the two countries.

U.S. State Department spokesman John Kirby said he shared de Mistura's concerns about the increased violence jeopardizing the process and urged Russia to press the Assad regime "to fulfill its commitments."

Staffan de Mistura, centre, attends a round of negotiations between the Syria's regime-tolerated opposition (Moscow - Cairo group) and the United Nations at the European headquarters of the United Nations in Geneva, Switzerland, on Wednesday. The peace talks are in 'great danger,' says de Mistura. (Martial Trezzini/Reuters)

The talks foundered last week after the Western- and Saudi-backed opposition called the High Negotiating Committee suspended formal participation in the indirect talks with Assad's envoys to protest alleged government cease-fire violations, a drop in humanitarian aid deliveries and no progress in winning the release of detainees in Syria.

De Mistura suggested back-channel work between Moscow and Washington was already under way to help the truce, and that a revival of it would help bring the two sides back to the table.

"I know that both the Russian Federation and the U.S. are talking among themselves on how to salvage on what has been actually a remarkable success — but needs to be sustained," he said, referring to the cease-fire. If it is revived, he predicted: "It will not be difficult for everyone to come back around the table."