The militants, from the Taliban group which controls much of southern and eastern Afghanistan, forced the Russian Ilushin-76 cargo plane to land last Thursday. They were holding the plane and its seven-member Russian crew in the southern city of Kandahar, authorities said.
It was not clear how they forced the aircraft to land, but apparently nobody was injured.
The aircraft's cargo of ammunition was for the government of President Burhanuddin Rabbani, a government spokesman said. The spokesman, who spoke on condition of anonymity, said the ammunition was purchased from Albania.
The Taliban, who are trying to unseat Rabbani and set up strict Islamic law, claim Rabbani has been receiving weapons from Afghanistan's old enemies: Russia and India.
Islamic insurgents threw out an Indian-backed communist regime in 1992, and the groups almost immediately began fighting each other in successive battles for power.
A spokesman, who identified himself only as Sergei, said in Kabul that he was negotiating for the release of the plane's crew in Kandahar, several hundred kilometers to the south. He declined to name the charter company he worked for.
"Negotiations are continuing for the release of the plane and its crew," he said. "We had an official contract with the Afghan government ... They [Taliban] have hijacked the aircraft."
He said one of the crew was having heart problems, but that the Taliban refused to release him.
According to Taliban spokesmen in Kandahar, the aircraft was carrying 3.4 million bullets and anti-aircraft ammunition.
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