LONDON - The Cartagena, an oil tanker owned by Libya’s government, entered the rebel-held port of Benghazi yesterday after reportedly being seized by opponents of leader Moammar Khadafy.

NATO forces hailed the tanker, found it was not breaking an arms embargo, and allowed it to enter the eastern port, a spokesman for the alliance said from Naples, declining to be identified by name in accordance with policy.

The 600-foot ship was boarded by Libyan rebels Tuesday with the help of a European government, Petroleum Economist reported, citing an unidentified person familiar with the operation.

“It’s quite a coup for the rebels,’’ said Samuel Ciszuk, a senior Middle East and North Africa energy analyst at research company IHS Energy in London. “It shows rebels being regarded as rightful representatives of Libya, as reports suggest they appear to be doing it with the understanding of NATO.’’

GNMTC, a shipping line owned by the Libyan government, said in March that one of its tankers was hijacked by an armed group believed linked to rebels. The uprising against Khadafy began in February, and the United States and other European allies have recognized the rebel Transitional National Council as the nation’s sole legitimate governing authority.

The tanker last signaled its position about 16 miles off Malta late Tuesday, according to ship-tracking data. Earlier, it was off the Algerian coast. Its expected destination was the city of Annaba in northeastern Algeria.

The tanker typically carries so-called clean petroleum cargoes, which includes products such as diesel, gasoline, and jet fuel.

The Libyan government has struggled to maintain fuel supplies since the fighting began. The country, once Africa’s third-largest crude producer, normally got four shipments of diesel and eight gasoline cargoes a month. One cargo contains about 9 million gallons, enough to fill about 650,000 cars.

Libya’s refineries will probably process no more than 90,000 barrels of oil a day this summer, compared with 370,000 barrels normally, according to the International Energy Agency, a Paris-based adviser to industrial nations.

In a separate development yesterday, a rebel field commander said opposition fighters are regrouping for a major offensive and hope to reach Khadafy’s stronghold, the capital of Tripoli, before the end of Ramadan in late August.

The rebels in Libya’s western Nafusa mountain area are receiving reinforcements, including volunteers arriving from Tripoli and other areas still under Khadafy’s control, said the commander, Muktar al-Akhdar. He spoke after an hours-long strategy meeting of unit commanders in the garrison town of Zintan, base of the area’s rebel command.

With fighting largely deadlocked for months, Libya’s rebels believe the Nafusa mountain front line is their best chance for striking the capital. But obstacles like land mines spotted in front-line areas and gasoline shortages have impeded progress, Akhdar said.

There is also the worry that Khadafy loyalists may be infiltrating the ranks, he said.

Neither side in Libya’s civil war has made significant gains for months. Khadafy controls Tripoli on the Mediterranean coast and towns around it, while rebels hold the east of the country and two pockets in the west: the Nafusa mountain range and the port city of Misurata.

A week ago, the Nafusa rebels launched a limited offensive, descending from their mountain plateau into the coastal plain and seizing three small towns. However, the advance has since stalled, with Khadafy’s forces entrenched in several towns blocking the way to Tripoli.

One of the flash points of fighting has been the town of Tiji, about 150 miles from the capital along a major highway, where Khadafy’s forces have been entrenched for the past week.

“We are trying to regroup and reorganize our troops in preparation for a major offensive, to march forward toward the coast, toward Tripoli and Zawiya and other areas along the coast,’’ Akhdar said. “Our preparations for the offensive are getting there.’’

Material from the Associated Press was included in this report.

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