A US airstrike near the Pakistan border killed the Taliban's southern military commander - an associate of Osama bin Laden and Taliban chief Mullah Omar, according to the US military. The death of Mullah Akhtar Mohammad Osmani, the highest ranking Taliban killed since 2001, regarded as one of three top associates of Omar, could hinder the militia's plans for an expected offensive early next year that would extend the surge in violence seen throughout Afghanistan in 2006, an analyst said. Osmani was killed Tuesday by a US airstrike while traveling by vehicle in a deserted area in the southern province of Helmand, the US military said. Two associates also were killed, it said. A Taliban spokesman denied the claim, though Afghanistan's Interior Ministry confirmed the killing, calling it "a big achievement." A US spokesman said the death was confirmed through multiple sources. Osmani, the Taliban's chief military commander in southern Afghanistan, played a "central role in facilitating terrorist operations" including roadside bombs, suicide attacks and ambushes against Afghan and international forces, the US said. "Mullah Osmani is the highest ranking Taliban leader we've ever killed," US military spokesman Col. Tom Collins said. "He was the chief of the Taliban's military operations, so his death is very significant and will hurt the Taliban's operations." Ahmed Rashid, a leading author on Islamic militancy, labeled the death a "major blow" to the Taliban. "It's the first casualty among the top Taliban leadership in the past five years, which makes the strike very significant," he said. The killing also comes ahead of what is expected to be a major Taliban offensive in the south in February or March, and Osmani may have been preparing for that when he was killed in Helmand, Rashid said. The Taliban militia has stepped up attacks this year, particularly in southern Afghanistan, and waged fierce battles with Western and Afghan forces. About 4,000 people have died in the violence, raising fears for the country's future and experiment with democracy after a quarter century of war. A purported Taliban spokesman, Qari Yousef Ahmadi, denied that Osmani had been killed, saying the airstrike instead killed Mullah Abdul Zahir, a group commander, and three other Taliban fighters. "I confirm that Osmani is alive and is in Afghanistan," Ahmadi told The Associated Press by phone from an undisclosed location. But Helmand provincial police chief Ghulam Nabi Malakhail said both Osmani and Zahir were killed in the airstrike, along with two other Taliban fighters. Collins said officials waited four days to announce the news in part so that they could be sure it was Osmani who was killed. "The vehicle was completely destroyed, there was nothing to recognize," Collins said. "But we have various intelligence assets that we monitor, that we look at very closely, and of course we work with the intelligence agencies of the Afghan government and through those sources we are sure that he is dead." Osmani was one of the first supporters of bin Laden among the Taliban and had received a lot of money from Arabs to build his military force, Rashid said. He was regarded as highly ideological and was instrumental in some of the excesses of the Taliban rule such as the destruction of the ancient Buddha statues in Bamiyan and the trial of Christian aid workers in 2001, Rashid said. As Kandahar corps commander during the Taliban regime, he would have been responsible for security around Omar. The whereabouts of Omar, the Taliban's reclusive leader who has a US$10 million reward on his head, remain a mystery. Collins said Osmani was part of a group of "co-equals" at the top of the Taliban leadership chain just under Omar and was also in charge of the Taliban's finances. "There's no doubt that it will have an immediate impact on their ability to conduct attacks," Collins said. "But the Taliban is fairly adaptive. They'll put somebody else in that position and we'll go after that person, too." Collins said Osmani had been "utilizing both sides" of the Afghan-Pakistan border, and that the US military had been tracking him "for a while." "When the time was right, and we thought we had a good chance of hitting him without causing any harm to civilians, we struck," he said. Interior Ministry spokesman Zemeri Bashary said the killing was "a big achievement, not only for international forces in Afghanistan, but for afghan forces too." Although the US said Osmani was an associate of bin Laden, Omar and Afghan insurgent leader Gulbuddin Hekmatyar, Collins said he didn't know the last time Osmani had contact with any of the three. Osmani was regarded as one of the top three Taliban leaders under Omar, along with another senior military commander in the south and southeastern regions, Mullah Dadullah, and influential policy-maker Mullah Obaidullah. In June, a man claiming to be Osmani - his face was concealed by a black turban - gave an interview to a Pakistani television network in which he said Omar and bin Laden were alive and well. He claimed to be receiving instructions from Omar.