BEIRUT — Syrian government forces advanced through southwestern Syria on Tuesday as tens of thousands of civilians fled ­Syrian and Russian airstrikes, ­piling onto trucks and heading deeper into rebel-held territory.

With violence escalating, diplomats and experts warn that the pocket could become a geopolitical tinderbox capable of destabilizing neighboring Jordan and triggering a wider conflict between Israel and Iran.

A cease-fire agreement between the United States, Russia and Jordan had largely kept the peace for months while the Syrian army focused on rebel-held regions closer to the capital, Damascus. 

But with those conquered, Syrian President Bashar al-Assad’s forces have turned their attention to Syria’s southwest, with help in recent days from Russian airstrikes.

Local activists uploaded video June 24 from Daraa, Syria, where Syrian state forces and their allies carried out airstrikes on opposition-held areas of Daraa. (Nabaa)

On Tuesday, the Britain-based Syrian Observatory for Human Rights said that forces loyal to Assad had taken control of the town of Busra al-Harir and the nearby Lajat area, prompting an exodus of families and cutting the rebels’ stronghold in half. 

“Warplanes and helicopters continued hovering in the skies above Daraa province,” said the monitoring group’s director, Rami Abdulrahman, who goes by a pseudonym. He placed the number of airstrikes in the hundreds.

Relief workers said hospitals also had been targeted. According to the Union of Medical Care and Relief Organizations, which supports medical staff in opposition areas, an ambulance driver was killed Tuesday in Busra al-Harir as he ferried patients to a clinic. 

“Nothing can justify his killing,” said Ghanem Tayara, the organization’s chairman. “It is beyond comprehension that after six years medical workers are still being killed with impunity.”

The Trump administration has frequently cited the southwest cease-fire, which came into force last summer, as evidence that Russia can make and adhere to agreements. But it remains unclear whether Moscow has the ability or willingness to impose its will on the Syrian government and its Iranian and militia allies.

The United Nations said Tuesday that at least 45,000 people have fled government advances in recent days, a figure that officials said could double as the fighting intensifies. But Jordan — already home to almost 700,000 registered Syrian refugees — said it would keep its border closed. 

Local activists and a doctor described the surrounding areas as ghost towns. Much of the population has been displaced, and those left behind are hiding in basements, they said.

“This heavy bombing has caused huge destruction,” said Emad, a medical worker who spoke on the condition that his full name not be used because of security concerns.

As Assad declared his intention to move into the southwest and initial skirmishes began, Israel and Jordan expressed alarm, fearing the presence of Iranian-backed militias along their borders, as well as a possible influx of refugees.

In a call last week to Russian Foreign Minister Sergei Lavrov, the State Department said, Secretary of State Mike Pompeo “reemphasized the U.S. commitment to the southwest cease-fire arrangement that was approved by President Trump and President Putin one year ago.

The statement said it was “critical for Russia and the Syrian regime to adhere to these arrangements and ensure no unilateral activity in this area.”

Although southern Syria was once a key zone of U.S. influence, the United States’ military and diplomatic sway has waned significantly in recent months, and Russia has emerged as a key power broker. Rebel groups once backed by Washington said last week that they have been told not to expect American military assistance. 

On Tuesday, a spokesman for Russia’s Defense Ministry blamed Syria’s rebels for the worsening humanitarian situation, “despite the efforts being undertaken” by Moscow. Russian broadcaster RT reported Tuesday that the country’s deputy foreign minister, Mikhail Bogdanov, would meet Jordanian officials in Moscow to discuss the crisis. 

King Abdullah II of Jordan is in Washington this week and is expected to discuss the fate of Syria’s south with American officials. 

Karen DeYoung in Washington and Amie Ferris-Rotman in Moscow contributed to this report.