This Tuesday, June 26, 2018 photo provided by Nabaa Media, a Syrian opposition media outlet, shows civilians inspecting damaged buildings that were hit by Syrian government forces bombardment, in Daraa, southern Syria. International aid organizations sounded the alarm Wednesday for the fate of thousands of desperate Syrians fleeing the onslaught of President Bashar Assad's forces in southwestern Syria, urging neighboring countries to take them in. (Nabaa Media, via AP)

This Tuesday, June 26, 2018 photo provided by Nabaa Media, a Syrian opposition media outlet, shows civilians inspecting damaged buildings that were hit by Syrian government forces bombardment, in Daraa, southern Syria. International aid organizations sounded the alarm Wednesday for the fate of thousands of desperate Syrians fleeing the onslaught of President Bashar Assad's forces in southwestern Syria, urging neighboring countries to take them in. (Nabaa Media, via AP)

BEIRUT (AP) — Airstrikes pounded rebel-held areas in southwestern Syria on Thursday, killing at least 17 civilians in an underground shelter and driving thousands from their homes, as scores of displaced people protested near the Israeli-occupied Golan Heights, demanding international protection.

The Syrian government pressed ahead with its offensive to reclaim the strategic region that extends along the border with Jordan and the Golan Heights, and which was until recently part of a U.S.-backed and negotiated truce.

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Signaling that the humanitarian crisis is likely to deepen, U.N. officials said that because of the fighting, no aid has entered from Jordan to reach the estimated 50,000 people displaced since Tuesday. Jordan, which is already hosting 660,000 registered refugees, says it cannot accept any more and has sealed its border, despite appeals from aid groups.

Near the Golan Heights, scores of newly displaced raised banners in protest. Thousands have fled to the area, saying they thought the proximity to Israeli forces would deter Syrian air raids. One activist said the camps are about three kilometers (2 miles) from the frontier.

Activists and a U.N. official said a new group fled their homes Thursday in the western Daraa province, where government strikes pounded Nawa, one of the largest towns in the area.

Linda Tom, spokeswoman for the U.N.’s Office for the Coordination of Humanitarian Affairs, said unconfirmed local reports suggest a “very large number” — estimated at 50,000 people— were driven from Nawa.

Yasser al-Khatib, of the activist-operated Nabaa news, provided a similar estimate.

A top U.N. adviser on humanitarian aid for Syria, Jan Egeland, said he believes the frontier with the Israeli-controlled Golan Heights is “hermetically closed.”

“I think you have to ask the Israelis whether they are going also to take part in giving shelter, protection, to people who flee,” Egeland said.

The Israeli military said it was monitoring the situation and was prepared for a variety of scenarios. An Israeli official said authorities are likely to provide humanitarian aid to fleeing Syrians, but not allow them to enter Israel. The official spoke on condition of anonymity because the plan was still evolving and he was not authorized to discuss the matter with the media. Israel has sent humanitarian aid across the border for several years. It has never confirmed claims that it provides military aid to Syrian rebels.

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The strategic southwestern corner of Syria had been under a so-called de-escalation agreement reached between Russia, the United States and Jordan in July of last year, but the truce has unraveled in recent weeks. Moscow is a key ally of Syrian President Bashar Assad, and a Russian air campaign launched in 2015 has helped him retake large areas of the country.

The Britain-based Syrian Observatory for Human Rights said warplanes pummeled rebel-held areas with as many as 240 raids, with crude barrel bombs dropped throughout the day. The Observatory described Thursday’s airstrike in the town of al-Musayfrah in eastern Daraa, which left 17 dead, as the worst violence since the government offensive began in the area on June 19.

The Observatory said at least five children were among the killed. An activist with the opposition-operated Horan Free Media, who goes by the name Abu Mahmoud Hourani, said the rescuers were still pulling bodies from the underground shelter by early afternoon.

He put the death toll at 20, saying women and children were among the casualties. “The situation on the ground is disastrous,” Hourani said.

Syrian government troops are seeking to dislodge rebels who have been in the area for years and to gain control of the commercial border crossing with Jordan.

In recent days, government forces have gained new ground, bisecting rebel-held territories in Daraa’s east and west and severing their supply lines.

The Syrian Civil Defense, volunteer first responders, said more than 150 airstrikes have targeted 12 towns and villages in Daraa since dawn.

Jordan has asked the U.N. to provide security for displaced Syrians within their own country, while Israel has not commented on the offensive or the displacement.

The Observatory said at least 46 civilians, including 15 children, have been killed since the offensive began, including the victims of Thursday’s airstrike.

The international aid organization CARE said humanitarian workers in the south are struggling to deliver basic supplies. One aid worker in an organization supported by CARE was killed in shelling on Tuesday.

“Civilians are paying the price of another military offensive,” said Wouter Schapp, CARE’s Syria country director. “Cities and towns are bombed daily, people are being uprooted and lack basic human necessities, such as water and shelter.”

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Associated Press writer Joe Federman in Jerusalem contributed to this report.