The United Nations mediator on Thursday, April 28 called on the leaders of the United States and the Russian Federation to salvage the "barely alive" two-month-old ceasefire in Syria and revitalize the damaged peace process, Reuters reports.

UN Special Envoy Staffan de Mistura voiced deep concern at the truce unraveling in Aleppo and at least three other hotspots, although he saw some narrowing of positions between the government and opposition visions of political transition.

"Hence my appeal for a U.S.-Russian urgent initiative at the highest level, because the legacy of both President Obama and President Putin is linked to the success of what has been a unique initiative which started very well. It needs to end very well," de Mistura told a news conference.

The United States and Russia must convene a ministerial meeting of major and regional powers who compose the International Syria Support Group (ISSG), he said, according to Reuters.

"There is no reason that both of them which 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 have created and which is still alive but barely."

The main opposition High Negotiations Committee (HNC) pulled out of the formal talks a week ago, in protest of intensified fighting and slow aid deliveries.

"How can you have substantial talks when you have only news about bombing and shelling? It's something that even I find it difficult, can you imagine the Syrians?" de Mistura said, adding he aimed to resume talks in May, though he gave no date.

Bashar Ja'afari, who led the government delegation, said on Tuesday the round had been "useful and constructive." But he gave no sign of ceding to the HNC's central demand for a political transition without President Bashar al-Assad.