The U.N. envoy for Syria is appealing to the U.S. and Russia to intervene "at the highest level" to help revive ailing Syrian peace talks without setting a date for them to resume.

Staffan de Mistura spoke to reporters early Thursday after briefing the U.N. Security Council via videoconference about the largely stalled talks. He said he hoped talks could resume sometime in May but suggested it would first require a reduction in fighting.

The briefing came at the end of a third session of talks this year.

The opposition suspended its formal role in the indirect talks with President Bashar Assad's government last week to protest alleged government cease-fire violations, a drop in humanitarian aid deliveries and no progress in winning the release of detainees in Syria.