BRUSSELS – NATO warships have intercepted several boats laying anti-shipping mines outside the harbor of the Libyan city of Misrata, military officials said Friday.

Alliance airstrikes, meanwhile, had also destroyed or damaged 600 targets in Libya since NATO’s aerial operations were launched a month ago, said British Brig. Rob Weighill, director of NATO operations in this North African country.

Misrata has been under siege by forces loyal to Moammar Gadhafi for several weeks, and though rebels have managed to expel regime forces from the city itself, the enclave is isolated and remains dependent for much of its food and other supplies on the sea link with the rebel capital of Benghazi. It appeared to be the first time sea mines have been used in the Libyan conflict.

“We have just seen Gadhafi forces floating anti-ship mines outside Misrata harbor today,” Weighill said.

“It again shows his complete disregard for international law and his willingness to attack humanitarian delivery efforts,” he said, speaking via teleconference from the operation’s headquarters in Naples.

Under international law, nations laying naval mines must alert shipping about their general locations to avoid accidents.

The sea mines were laid 1 to 2 miles offshore by deliberately sinking inflatable boats on which they were being carried, said a NATO statement released later in the day. It said three mines had been found and disarmed.

“NATO forces are now actively engaged in countering the mine threat to ensure the flow of aid continues,” said Italian Navy Vice Adm. Rinaldo Veri, who is responsible for NATO’s maritime forces off the Libya coast.

It has been a month since NATO assumed control of the U.S.-led military operation in Libya, under a U.N. mandate to protect civilians from attack.

Since then, alliance warplanes have conducted 4,242 sorties, of which 1,766 were strike sorties — which does not necessarily mean they identified their targets and deployed their weapons. In addition, 19 NATO ships are patrolling the central Mediterranean.

Weighill said the targets hit during the aerial onslaught include 220 tanks and armored personnel carriers, 200 ammunition facilities and 70 surface-to-air missile systems.

“NATO has already demonstrated the extent to which we are versatile and able to switch focus depending on where the major threats to civilian populations are,” he said, adding that the alliance is now also focusing on Zintan and Yefrin, two towns southwest of the capital of Tripoli where pro-Gadhafi forces were conducting offensive operations.

Meanwhile, a NATO spokeswoman said rebel chief of staff Abdel-Fatah Younes met Thursday with Martin Howard, the alliance’s assistant secretary-general for operations, and a representative of the international military staff in Brussels. Carmen Romero said there had been “an exchange of views on Libya,” but declined to elaborate.

Younes, a former Libyan interior minister who defected to the opposition, called on the international community to supply heavy weapons to the rebels.