NAPLES, Italy — NATO warplanes attacked eight Libyan ships on Thursday night in three coastal locations, including the port of Tripoli, expanding the air campaign against what allied officials said was an increasing seaborne threat from Moammar Gadhafi’s forces.

On Monday, allied warships thwarted an effort by Gadhafi loyalists to use small inflatable boats packed with high explosives to threaten ships carrying relief supplies to the contested port city of Misrata, 130 miles east of Tripoli, the capital.

That episode was the third time in recent weeks in which NATO forces had confronted pro-government maritime forces off the Libyan coast, after intercepting boats laying mines in Misrata’s harbor on April 29 and defeating an attack by small boats on the port last week.

The allied attacks late Thursday against Libyan vessels in the ports of Tripoli, Al Khums and Sirte were the first time in the two-month-old air campaign that the alliance had carried out planned airstrikes against Libyan ships, military officials said. NATO warplanes have previously returned fire at Libya ships that shot at them.

“All NATO’s targets are military in nature and are directly linked to the Gadhafi regime’s systematic attacks on the Libyan people,” Rear Adm. Russell Harding, the deputy commander of the NATO mission, said in a statement. “Given the escalating use of naval assets, NATO had no choice but to take decisive action to protect the civilian population of Libya and NATO forces at sea.”

Harding said the eight vessels attacked were all “naval warships with no civilian utility.”