Washington - The US-led coalition toppled the Taliban with relative ease following the September 11 terrorist attacks, but seven years later the United States and its NATO allies are in a tough fight with the resurgent Islamic militants.

After years of taking a back seat to the conflict in Iraq, Afghanistan has re-emerged as a major national security concern and taken a central role in the November presidential campaign.

The US military has been scrambling to get more troops into the fight, while Washington has urged NATO allies in Europe to provide more soldiers to counter sharply rising violence, which has lately rivaled or even surpassed the bloodshed in Iraq.

On Wednesday the top US military officer raised questions about the prospects for success in Afghanistan under the current strategy.

"I'm not convinced we're winning it in Afghanistan," Admiral Michael Mullen, chairman of the Joint Chiefs of Staff, told the House Armed Services Committee.

Mullen said he was ordering a review of the current strategy and seeking a more comprehensive approach, including a way to counter Taliban rebels' use of Pakistan's remote, mountainous frontier regions to mount crossborder attacks on coalition forces.

Mullen warned that victory will not be achieved until the international community invests more in Afghanistan to develop infrastructure and train teachers, judges and lawyers.

"We cannot kill our way to victory," Mullen said.

Democratic hopeful Barack Obama, however, is hoping the prolonged fight will help to propel him to victory in November. Obama has badgered US President George W Bush for his decision to invade Iraq in March 2003, before finishing off the Taliban and al-Qaeda in Afghanistan.

Obama has said he will begin withdrawals of US troops from Iraq and shift forces to Afghanistan, which he contends is the "central front" in the war on terrorism, a reference to Bush and Republican opponent John McCain's description of Iraq.

Bush, acknowledging the urgency of the situation, announced Tuesday that a Marine battalion that had been slated to head to Iraq in November will now be diverted to Afghanistan.

"As we learned in Iraq, the best way to restore the confidence of the people is to restore basic security, and that requires more troops," Bush said.

Obama has sought to closely pin McCain to Bush's policies in Iraq and Afghanistan, charging that both men have neglected the conflict in the latter by focusing on Iraq.

"What President Bush and Senator McCain don't understand is that the central front in the war on terror is not in Iraq, and it never was," Obama said Wednesday. "The central front is in Afghanistan and Pakistan, where the terrorists who hit us on 9/11 are still plotting attacks seven years later."

McCain has characterized Obama plan's to pull soldiers out of Iraq as "reckless" and maintains that the United States is capable of prevailing in both conflicts.

"It is clear that we need additional forces in Afghanistan, and I support the new deployments," McCain said. "Senator Obama believes we must lose in Iraq to win in Afghanistan. I want to win in Iraq and in Afghanistan."