BENGHAZI, Libya – A NATO airstrike in the besieged rebel-held city of Misrata mistakenly killed 12 Libyan rebels, an official with the transitional government confirmed Thursday, while new fighting was reported on Libya’s western border with Tunisia.

The strike on Wednesday was at least the third in which rebels were killed since NATO fighter jets began pounding forces loyal to Moammar Gadhafi more than five weeks ago in a mission to protect Libyan civilians.

Leaders of the anti-Gadhafi forces have called the incidents unfortunate accidents in a worthy cause, reflecting wide support in rebel ranks for the NATO attacks.

Nonetheless, both NATO and the rebels have said additional precautions — such as marking rebel vehicles and reporting precise positions to NATO — have been employed to avoid more rebel deaths.

“It is regrettable, but we know the people of Misrata understand,” said Shamsiddin Abdulmolah, a media liaison with the transitional council who confirmed the rebel deaths. “We know NATO didn’t do this on purpose.”

There was no immediate response from NATO.

NATO’s bombing is widely considered to have prevented Gadhafi forces from retaking rebel-held territory, including the eastern city of Benghazi, de facto capital of the opposition. Rebel leaders have urged their international allies to step up the air campaign against the dictator’s regime.

Two previous inadvertent NATO strikes on anti-Gadhafi forces killed at least 18 rebels and injured many more.

Reports from the scene indicated that Wednesday’s strike was near Misrata’s port, which has been a lifeline for supplies into the city and refugees going out. NATO has been bombarding loyalist forces shelling the port area.

In weeks of difficult battle, rebel fighters managed to dislodge Gadhafi forces from the center of Misrata, commanders say, but regime loyalists based outside the center continue pounding the town and its port with artillery shells and rockets.

Three more people were killed Thursday in rocket or mortar attacks on Misrata, said a doctor contacted via Internet telephone. Seven were killed on Wednesday, he said.

Meanwhile, in far western Libya, pro-Gadhafi forces retook a strategic border post along the Tunisian frontier that rebels captured a week ago, the Reuters news agency reported. Libyan soldiers hoisted their flag at the Dehiba-Wazin border crossing, said Reuters, which reported that fighting had also broken out inside Tunisia.

The government in Tunis did not comment on the report. Tunisia has taken in thousands of Libyans who have fled the fighting in their homeland.

Fighting was also reported in the remote southeastern town of Kufra, where, Reuters said, Libyan state television asserted that the government had seized control. The rebels denied that the town had fallen.