Kerry and other diplomats are left striving for signs of progress on Syria

VIENNA — Diplomats trying to revive faltering Syrian peace talks on Tuesday called for air drops of humanitarian supplies to blockaded towns and issued a stern warning to opposition rebels not to violate the truce or they might not be protected from future airstrikes.

After a meeting of diplomats from two dozen countries and organizations, U.S. Secretary of State John Kerry said they want to convert a fragile, often-violated truce into a fullfledged nationwide ceasefire but acknowledged that it was a difficult goal when both the government and some rebel groups had taken actions to undermine it.

“We can’t give vetoes to bad actors or avoid the consequences for any side’s actors who have an agenda that is different from that of reaching an agreement and trying to make peace,” Kerry told reporters.

In a statement, the diplomats set a June 1 deadline for deliveries of food and medicine to more than a dozen towns and neighborhoods where U.N. relief agencies have been turned away, usually by the government.

If relief aid does not get through overland by that date, they urged World Food Program to conduct air lifts, which have taken place elsewhere in Syria, though with mixed success.

In February, the U.N.’s World Food Program said Syrian aid workers on the ground recovered fewer than half of the 21 pallets dropped over the eastern city of Deir al-Zour by Russian planes in a first attempt to reach the besieged region.

Another WFP-directed airdrop last month, however, managed to get nearly all the 22 pallets to the Syrian Arab Red Crescent, the agency said.

In order to stop the violence and truce violations, Kerry said the countries in the International Syria Support Group are prepared to consider taking harsher steps against groups that have “engaged in a pattern of persistent noncompliance.” One step, he said, could be dropping them from the protective umbrella that comes with the truce.

The truce agreed upon in February as a confidence-building measure ahead of the U.N.-sponsored peace talks in Geneva all but collapsed earlier this month with fierce fighting around Aleppo.

Under the terms of the U.N. Security Council resolution establishing the talks, only two groups designated as terrorists — the Islamic State and Jabhat al-Nusra, the Syrian branch of al-Qaida — can be legitimately attacked under the truce.