Secretary of State John F. Kerry, left, and Iran's Foreign Minister Mohammad Javad Zarif, right, attend crisis talks on Syria in Vienna on May 17. (Pool photo by Leonhard Foeger /via European Pressphoto Agency)

Diplomats trying to revive faltering Syrian peace talks on Tuesday called for airdrops of humanitarian supplies to blockaded towns and warned rebels that they might not be protected from future airstrikes if they violate an ongoing partial truce.

After a meeting of diplomats from two dozen countries and organizations, Secretary of State John F. Kerry said the goal is to convert the often-violated truce into a full-fledged nationwide cease-fire, but he acknowledged the difficulty of doing so.Both the government and some opposition groups have taken actions that undermine the ongoing truce.

“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 said at a news conference.

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

U.S. Secretary of State John Kerry speaks during a news conference in Vienna, Austria on May 17. (Leonhard Foeger/AP)

If relief does not get through over land by that date, they urged the U.N. World Food Program to conduct airlifts, which have taken place elsewhere in Syria, with mixed success.

In February, the World Food Program said Syrian aid workers 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 in the area, however, managed to get nearly all its 22 pallets to the Syrian Arab Red Crescent, the agency said.

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

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

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

That has been difficult to enforce, however, because some of the rebel groups that are part of the talks have fought in the vicinity of, and occasionally alongside, those two groups, allowing the government to say that it was targeting terrorists even when striking opposition groups involved in the talks.

In a particularly worrisome incident, Ahrar al-Sham, which is part of the talks, reportedly joined with Jabhat al-Nusra in an attack on the Alawite village of Zara last week, killing 19 people, including civilians. Alawites are a minority group that form the backbone of the regime.

Ahrar al-Sham is backed by Qatar, a member of the Syria support group.

The diplomats urged all opposition groups to separate themselves, “physically and politically,” from the Islamic State and Jabhat al-Nusra.

Kerry said the diplomats agreed to use their influence — Russia and Iran with the Syrian government, and the United States and everyone else with the opposition — to prevent violations so the truce can be expanded.

Kerry sounded exasperated at the infighting that has marked the talks.“A variety of competing interests are going to have to be reconciled” for the conflict to end, he said. “Those involved in this conflict with competing agendas are going to have to prioritize peace.”

The diplomats came to Vienna with little hope of a breakthrough in the Geneva negotiations between the government and the opposition. The talks have barely gotten off the ground, with the parties never even meeting in the same room. The last round was in April, and no date has been set for another round.

U.N. envoy Staffan de Mistura, who is leading the talks, said the negotiations could resume when there is “some kind of concrete outcome” from Tuesday’s meeting, such as a cease-fire for humanitarian access.

But the huge gaps in positions, not only between the government and the opposition but within the International Syria Support Group, could not be papered over.

Russian Foreign Minister Sergei Lavrov took issue with Kerry’s characterization of Russian support for the government of Syrian President Bashar al-Assad.

“We don’t support Assad,” Lavrov said through a translator. “We support the fight against terrorism. Today, we don’t see any more real and efficient force than the Syrian army.”

Opening a small window in the talks, he said some of the diplomats attending the meeting had told him that the choice was between a fight against terrorism and a fight against the Syrian regime.

“Wrong,” he said, declaring that the choice was between priorities. “Everybody acknowledged that Assad’s regime is the lesser evil for them than if we compare it with increasing chaos if there is no political process.”

Although there is a broad agreement on a transition sometime in the future, the outlines are vague.

The United States and the opposition say Assad must go. The Syrian government envisions a national unity government that could include some opposition members — presumably with Assad still in power. It considers many of the opposition groups to be terrorists, however.

The diplomats are facing an August deadline to create a framework for a political transition. Kerry indicated, however, that the deadline was flexible so long as progress was being made.

As the talks teeter between promise and stalemate, the bloodshed continued. Human Rights Watch has asked the United States and Russia to investigate the attack on Zara and a May 5 airstrike on a camp for 4,500 displaced people near the Turkish border in which at least 20 people died.