Oppositions forces in northern Afghanistan say they have broken through Taleban defenses and entered the strategically important northern city of Mazar-e-Sharif. Taleban officials admit their forces have retreated from parts of the town.

Unidentified Taleban officials have told a Pakistan-based Afghan news agency, Afghan Islamic Press, that forces of the opposition Northern Alliance have entered southern parts of Mazar-e-Sharif. They say the opposition's advance on the city comes after heavy American bombing of their defenses in the area.

Leaders of anti-Taleban alliance say their forces have taken control of the airport outside the city and that Taleban soldiers are fleeing towards the capital, Kabul. Both sides are said to have suffered heavy casualties in the fighting, but no figures are available.

Reports from the war front could not be independently verified.

The fall of the key northern Afghan city would be the most significant gain by anti-Taleban forces since the American air strikes in Afghanistan began nearly five weeks ago. It will also open up vital supply lines to the opposition forces from neighboring Uzbekistan. The central Asian republic is a close ally in the U.S. bombing campaign, which is meant to punish the Taleban for refusing to surrender Osama bin Laden and his Al-Qaida network.

Meanwhile, hundreds of fighters of the Northern Alliance, backed by tanks and artillery, are seen preparing for a major attack just north of Kabul. The anti-Taleban alliance is widely feared by the residents in the city because of a bitter struggle over Kabul in the early 1990's that killed tense of thousands of people.

In light of reports that the opposition forces have made significant advances near Mazar-e-Sharif, Amnesty International has called on the anti-Taleban Northern Alliance to ensure respect for humanitarian law and refrain from revenge killings. In a statement, the human rights group says both the Taleban and those opposed to them have been responsible for grave abuses in the past, including indiscriminate killings of civilians and ethnically targeted killings.