BEIRUT — Syrian government forces reached a crucial crossing on the border with Jordan on Friday, state media said, as their long-heralded push through the opposition’s southern stronghold appeared to be in its final stages and rebel forces negotiated surrender.

Syria’s state-run news agency, Sana, reported that the country’s red-white-and-black flag had been raised above the Nasib crossing, a sign that for the first time since 2015, President Bashar al-Assad’s forces have regained control of the vital economic artery.

The Trump administration had presented a year-long cease-fire across the zone as the jewel in the crown of U.S.-Russia relations. But that lull was shattered by a government offensive that has succeeded in recapturing within just a few weeks most of the opposition-held southwestern pocket, famed as the heartland of Syria’s anti-government rebellion, and leaving rebels and civilians there decrying what they see as abandonment by Washington. 

On Friday, rebel negotiators said they were on the verge of a deal that would see them hand over their heavy weapons in return for a cease-fire and, for some, offer safe passage to the opposition’s last bastion, in the north. 

“Today we are negotiating to stop the barbaric assault,” said Ibrahim al-Jabawi, a spokesman for the rebels’ negotiating team. “We are back at the table to ensure the safety of our people.”

Local residents said Friday that a cease-fire was in effect across much of the region. According to the United Nations, as many as 325,000 people have fled the fighting in less than three weeks, most of them heading toward the closed borders with Israel and Jordan.

The bombing they fled damaged homes, hospitals and aid depots. “We tried to go back and collect our furniture,” said one woman whose town had been subjected to heavy bombing. “There was nothing left. Our lives there are finished now. It’s just bombshells and rubble.”

The Syrian Observatory for Human Rights monitoring group said that the area around the Nasib border crossing is under the control of Syrian soldiers and Russian military police. Although Jordan has openly opposed Assad’s government throughout much of the seven-year Syrian conflict, arming opposition fighters and taking in more than 655,000 refugees, it moved to normalize relations last year.

Trade between the two countries is expected to resume once the Nasib crossing is formally opened to commercial traffic.

Eyewitnesses in the area said that a dozens-strong convoy of armored vehicles and tanks flying Russian and Syrian flags was seen driving toward Nasib late Friday afternoon. 

Although rebel officials framed their capitulation as a result of pragmatism, an effort to keep civilians safe, others interviewed Friday said that the legacy of earlier major battles contributed to the diminishing morale of opposition fighters and civilians in the area. 

“We have seen what the regime does to the last territories it hasn’t conquered,” said one Syrian aid worker who spoke on the condition of anonymity for fear of retribution against her family by Syrian government forces. “We’ve seen the same story over and over again. The areas that don’t negotiate are bombed into submission, and we couldn’t accept that. This war needs to end, no matter what the ending looks like.”

For Assad, victory in southwestern Syria would spell the end of rebel control in areas that do not have a major foreign sponsor. Once the Syrian government clears the M5 highway that runs from Nasib to the capital, Damascus, and on to the northern city of Aleppo, it will finally have regained control of the country’s most important thoroughfare.

Aid groups and medical workers had accused Assad’s forces of using chemical weapons in the fight to regain the Damascus suburbs, and on Friday, a global watchdog concluded that chlorine was indeed used in the city of Douma a day before rebel forces surrendered there. 

The widely reported allegations of a chemical attack on April 7 prompted the Trump administration to launch military action against several Syrian military installations associated with chemical weapons production, in conjunction with France and Britain.

In an interim report released Friday, the Organization for the Prohibition of Chemical Weapons said its inspectors had discovered traces of “various chlorinated organic chemicals” across two sites it inspected. But in the case of a building in which videos and photographs showed dozens of men, women and children choking and foaming at the mouth, the report also said that the teams had not been able to access every room. 

“The FFM team was unable to gain access to some apartments,” it said, referring to members of the fact-finding mission. “The representatives of the Syrian Arab Republic stated that they did not have the authority.”

A fuller report is expected in the coming months. 

Zakaria Zakaria in Istanbul and Suzan Haidamous in Beirut contributed to this report.