Libyan rebels say forces loyal to Libyan leader Moammar Gadhafi have fired rockets at opposition fighters near the border with Tunisia.

The Reuters news agency quotes witnesses who say the Libyan rockets crossed the border into Tunisia, an act that could raise tensions that are already high between the two countries.

Libya fired rockets into Tunisia on May 17, and the Tunisian government threatened to report its neighbor to the U.N. Security Council for “enemy” actions. Libyan forces also have engaged in periodic short skirmishes with Tunisian troops after Libya's internal fighting spread across the border.

Meanwhile, Libyan rebels have advanced from the western port of Misrata towards the capital, Tripoli, breaking a government siege as fighters across the country renew their revolt against leader Moammar Gadhafi.

Several rebel units have pushed the front lines west from Misrata to the outskirts of Zlitan, a neighboring town held by Mr. Gadhafi's forces.

The two sides traded heavy rocket and artillery fire Monday, with rebels using arms seized from government weapons depots and fresh armaments shipped from the opposition stronghold of Benghazi.

Anti-government fighters in eastern Libya suffered a major setback Monday near the oil town of Brega, where pro-Gadhafi troops killed at least 21 rebels in an ambush east of the city.

U.S. Secretary of State Hillary Clinton called on African nations Monday to cut ties with Moammar Gadhafi and demand his removal.

Speaking at the African Union headquarters in Addis Ababa, Ethiopia, Clinton told African leaders gathered that Mr. Gadhafi has lost his legitimacy to rule. She urged them to call for a genuine cease-fire and throw their support behind the Libyan rebels' Transitional National Council.