(CNN) -- Government shelling of the seaside town of Misrata resumed Thursday afternoon, with heavy artillery and mortars targeting residential areas, killing 10 people, a member of the medical committee of the Misrata Council told CNN.

Also Thursday, rebels retook the border area of Dehaiba/Wazin after forces loyal to leader Moammar Gadhafi had taken it earlier in the day, a witness said.

The witness, Yousef Gaigi, said about 100 rebel vehicles equipped with assault rifles and antiaircraft weapons drove from the Libyan city Zintan to the border area, where they clashed with pro-government forces. "The fight resulted in many casualties," he said, without citing a number.

Gaigi said the pro-Gadhafi forces then fled to the Tunisian side of the border and handed their weapons to the Tunisian army.

The Tunisian Defense Ministry said the soldiers were taken farther north to the border at Ras Ajdir and sent back to Libya. Asked whether the Tunisians had returned the Libyans' weapons, a ministry official said, "It's possible."

Earlier Thursday, the Tunisian Foreign Ministry had expressed "deep concern" over the military escalation near the crossing its border with Libyan and demanded that the Libyan government "put an end to these violations," according to the state-run Tunis Afrique Presse (TAP).

The incidents occurred a day after the carnage in Libya continued to mount: on Wednesday, pro-government forces launched the heaviest shelling yet on the port of Misrata, much of which appeared to be a wasteland, rebels said.

Also Wednesday, a NATO airstrike on Misrata killed 11 rebel fighters and wounded two others, witnesses told a reporter, who saw the bodies.

From the time NATO began its operation on March 31 until Tuesday, the organization had conducted 3,981 sorties, of which 1,658 were strike sorties, according to an update issued Wednesday.

NATO is leading an international military operation in Libya that includes airstrikes targeting Gadhafi's military resources. It is operating under a U.N. Security Council resolution authorizing any means necessary -- with the exception of foreign occupation -- to protect civilians.

Misrata, the third-largest city in the North African country, has been hemmed in on three sides by Gadhafi's forces. Though rebels said they had gained control of the city's center and had pushed government forces outside the city, they said Gadhafi's forces were continuing to attack Misrata with heavy weaponry.

The port has served as a crucial route of escape and as a lifeline to humanitarian aid.

CNN's Frederik Pleitgen and Saad Abedine contributed to this report.