"We call on the international community to assume its legal and moral responsibilities and to arm, without further delay, the Libyan army," said Libya's representative to the Cairo-based Arab League, Ashur Bou Rashed.
More than three years after dictator Moamer Kadhafi was toppled and killed in a NATO-backed revolt, Libya is awash with weapons and powerful militias, and has rival governments and parliaments.
The internationally recognised government and parliament elected in June have been based in the remote east since an Islamist-backed militia coalition, Fajr Libya (Libya Dawn), seized Tripoli last year.
Bou Rashed insisted on the need to score a "military victory over the brutal militias to prevent them from further expanding their influence across Libya" and help reach a political solution.
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His comments came as the United Nations postponed peace talks it was hoping to hold today between Libya's warring factions.
The talks, originally slated for December 9, have been repeatedly delayed because of escalating violence.
Arab League chief Nabil al-Arabi voiced regret at the latest postponement.
"Our priority today is to adopt a decisive position that would lead to an immediate end to armed terrorist operations" in Libya, he said.
The Arab League, in a statement at the end of its Cairo meeting, condemned "all attacks on institutions and economic installations" in Libya, notably its oil sector.
In mid-December, Fajr Libya launched an assault to try to seize key oil terminals.
At least 22 pro-government forces were killed in fighting with the militias who fired a rocket that set ablaze tanks at Al-Sidra oil terminal. The military retaliated by raiding Misrata.
The tanker was hit yesterday after its refusal to stop for checks on its cargo raised suspicions, an armed forces spokesman said.
But the Greek government said the vessel was plying a longstanding route under contract to the Libyan state oil company, and demanded prosecution of those responsible for the deaths of the Greek and Romanian crewmen.