NATO says it has intercepted an oil tanker it had reason to believe was set to deliver fuel for use by Libyan leader Muammar Gaddafi's military forces.
The ship was intercepted on Friday hours after the alliance sunk eight Libyan warships in an attack said to be the broadest on Libya's naval forces since the alliance joined the conflict.
Gaddafi's government is seeking to raise fuel imports for military purposes and to keep civilian vehicles running in areas he controls.
International sanctions do not include a fuel embargo.
"NATO naval forces can deny access to vessels entering or leaving Libyan ports if there is reliable information to suggest that the vessel or its cargo will be used to support attacks or threats on civilians, either directly or indirectly," NATO spokeswoman Carmen Romero said on Friday.
NATO, working under a UN mandate to protect civilians from government forces, also said military and political pressure was weakening Gaddafi's hold on power in what appeared to be a marked escalation of a Western-led bombing campaign.
The overnight strikes hit the vessels in the ports of Tripoli, Al Khums and Sirte, and also hit a dockyard facility for launching the fast inflatable boats that Libyan forces have used for attacks around rebel-held Misurata.
"The destruction last night of the facility and a significant stockpile of the boats will reduce the regime's ability to sustain such tactics," Britain's Major-General John Lorimer said.
He said the port was the nearest concentration of regime warships to the port of Misurata, which Gaddafi has repeatedly attempted to close to humanitarian shipping.
Mohammed Rashid, general manager of the Tripoli port, told reporters the coastguard boats were used to patrol Libyan waters for immigrant boats trying to make it to Europe and for search-and-rescue activities.
The port official said some damage was done to the port, but it was minimal.
Fears over ships
A government official later said he feared the NATO strike would discourage ships from using the Tripoli port, reducing imports and driving up prices of basic goods for Libyans.
Reporters who toured the area from a distance said a warship could be seen on fire, with flames and plumes of smoke bellowing from the stricken vessel.
Rear Admiral Russell Harding, deputy commander of the NATO operation, said the Gaddafi regime was employing more ships in its campaign against rebel fighters.
"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," he said in a statement.
"NATO has constantly adapted to the rapidly changing and dynamic situation in Libya and at sea."
In Brussels, headquarters of NATO, video clips from the jets' gun cameras were played showing the bombing of two frigates and a port facility.
The two frigates, a Soviet-built Koni class anti-submarine boat and a French-built Combattante class missile craft, were moored at the dock when they were hit with laser-guided bombs.
It was not immediately clear whether their crews were aboard when they were struck.
"Our aim was not to destroy these ships but to remove their military ability," NATO spokesman Wing Cmdr. Mike Bracken told reporters. "They were identified as legitimate and legal targets."
Shelling, meanwhile,was heard in the Ghabat al-Qasr neighbourhood of the capital, Tripoli, in the early hours of Friday.
In some of the latest strikes, NATO hit Gaddafi's forces around 15km east of the opposition-held town of Zintan in the Western Mountains region. The town and the port city of Misurata have seen some of the heaviest fighting in recent weeks.
Al Jazeera also confirmed there was heavy fighting on the Tunisia-Libya border. Casualty numbers however are not yet known.
Three months into an uprising against Gaddafi's four-decade rule, rebels control the east and pockets in the west but the conflict has reached a stalemate as rebel attempts to advance on Tripoli have stalled.