A year-old cease-fire in southwestern Syria, long cited by the Trump administration as a test of whether the United States and Russia can work together in a conflict that finds them on opposite sides, is on the verge of collapse amid an accelerating offensive by the Syrian government and its allies.

The U.S. and Russian governments this week accused each other of failing to adhere to the agreement that was finalized in July 2017 at a meeting between President Trump and Russian President Vladi­mir Putin. Both governments said that Syria will be a main agenda item when the two leaders next meet, in Helsinki in ­mid-July.

In the meantime, the United Nations warned that what it described as the potential next bloodbath in Syria, this time along the borders of Jordan and Israel, risked a wider conflict.

On Sunday, Russian warplanes joined the offensive by forces loyal to Syrian President Bashar al-Assad to reclaim territory in the border province of Daraa, which had been captured by rebels over the past seven years. The attacks have targeted medical facilities and rescue workers, and prompted tens of thousands of residents to flee their homes for safer territory.

An airstrike Thursday killed at least 17 civilians who had been hiding in an underground shelter, according to witnesses and war monitors.

As thousands fled the intensifying bombardments, advancing Syrian government forces seized control of the town of Harak, according to a statement by the Lebanese Hezbollah militia, which is among the Iranian-backed groups taking part in the offensive.

Secretary of State Mike Pompeo said at a Senate hearing on Wednesday that Russia has violated the Trump-Putin agreement and that “we have both spoken to them about it and issued public statements indicating exactly that.”

Asked by Sen. Christopher A. Coons (D-Del.) whether the Russians were “listening, and do they care?” Pompeo said, “They’re certainly listening,” but that he “can’t speak to” whether they care.

“It’s not just our voice, it’s the voice of the Israelis and the Jordanians as well,” Pompeo said, calling Russian behavior “unacceptable.” He said that the 2,200 U.S. troops in Syria “don’t have the ability to reach the region” where the offensive is taking place. Those troops are deployed against the Islamic State in the eastern part of the country.

Jordan’s King Abdullah II visited Washington this week, in part for Syria consultations.

Israel’s army chief of staff, Gadi Eisenkot, arrived in Washington on Thursday, and that country’s national security cabinet is set to convene Sunday to discuss the border situation. Israeli aircraft have launch repeated strikes against Iranian targets inside Syria, and Prime Minister Benjamin Netanyahu’s government has said it will not tolerate any Iranian or allied forces in the border area.

In a message to U.S.-allied Syrian rebels in the area, the U.S. Embassy in Amman said they would be on their own if they decided to resist the government advances.

“You should not base your decision on the assumption or expectation of military intervention by the U.S.,” said the message, delivered in Arabic over a messaging app and shared by a member of the Syrian opposition.

Following a statement by the Russian Defense Ministry that it remained committed to the cease-fire deal, the Russian Foreign Ministry said Thursday that it was aiding the fight against “terrorists” in the region.

“It’s high time our American partners did what the Syrian forces are doing now with the support of Russia’s aerospace forces,” ministry spokeswoman Maria Zakharova told reporters.

U.S. officials have often referred to the southwest cease-fire as evidence that efforts to cooperate with Russia on both counterterrorism and a political solution to Syria’s separate civil war can succeed.

The fact that the agreement — in which fighters opposed to Assad and government forces froze in place along a line policed by Russia — largely held for nearly a year “is a significant achievement in itself,” Brett McGurk, the special U.S. envoy to the counter-Islamic State coalition, said at a conference in Israel last month.

As part of the deal, formalized by Trump and Putin at the Group of 20 summit in Germany last July, all “foreign fighters” were to leave the area, including Hezbollah and other Iranian-backed militias.

“One year ago, this southwest corner was on the verge of a major conflict, and Daraa City was at risk,” McGurk said. “Since then, the fighting between the opposition and the Syrian regime has largely stopped, hundreds of families have returned to their homes and thousands of lives have likely been saved.”

Just weeks later, however, Assad declared his intention to retake one of the last remaining parts of the country outside government control and began moving forces southward from around Damascus.

The fighting seems set to intensify despite appeals by the international community to allow time for diplomacy to avert a battle that risks triggering a renewed humanitarian catastrophe and could draw in regional countries. 

The U.N. special envoy for Syria, Staffan de Mistura, told the Security Council on Wednesday that he is “gravely concerned” by the battlefield developments. He urged world powers to find a solution that will spare civilian suffering and avert regional tensions so that “we do not see once again . . . a repetition of what we saw sadly in Aleppo and Eastern Ghouta.” He was referring to the devastating carnage in the city in northern Syria two years ago and in the Damascus suburb earlier this year.

“And yet I see things moving in this direction,” he said. Already, the fight for Daraa is turning into a grim replay of those key battles.

The Britain-based Syrian Observatory for Human Rights said at least five children were among 17 people known to have been killed in Thursday’s strike on the shelter in the rural town of Musayfra. The toll could rise, the group said, because many people are critically injured.

A doctor who joined the rescue teams that headed to the scene put the toll at 23, including 11 children, and said approximately 15 other people were taken to hospitals. He said about 40 people from four families had been hiding from relentless airstrikes in the basement of a building on the outskirts of the town when the building took a direct hit.

“There were pieces of flesh and rubble everywhere,” said the doctor, who spoke on the condition of anonymity out of fear for his safety.

The families had already escaped their homes in the center of the town in the hope of avoiding the bombardments, he said.

Photographs posted on social media by activists showed the bloodied corpses of seven children allegedly killed in the strike, five of them crumpled in the back of a van and two lying on the ground.

It was the bloodiest attack since Russian warplanes joined the offensive, and it came amid a steadily escalating onslaught of strikes in which hundreds of bombs fell on towns and villages Thursday, according to the observatory and residents.

Most of those fleeing the fighting have sought refuge closer to the Jordanian border and the demarcation line between Syria and the Israeli-occupied Golan Heights, but neither country is allowing the refugees to cross over, leaving them vulnerable to shortages of food and water in soaring summer temperatures, aid agencies say.

Aid delivery routes across the Jordanian border have been suspended because of the fighting, and 750,000 civilians are at risk, U.N. humanitarian coordinator Jan Egeland told reporters Thursday. “This is heart-wrenching because this was a zone where people felt safe until just days ago,” he said.

Sly reported from Beirut, and Zakaria reported from Istanbul.