KABUL : Afghan President Hamid Karzai vowed to combat corruption and invited his chief rival to join the government after he was sworn in for a second term on Thursday, facing Western pressure to restore legitimacy as a US-led war stretches into a ninth year.

Karzai took the oath of office with a Taliban insurgency killing record numbers of Western troops and Afghans and limiting government control in growing parts of the country after his fraud-tainted re-election.

"Corruption is a dangerous problem," he said in an address delivered before an audience of visiting foreign ministers, including US Secretary of State Hillary Clinton, at his heavily fortified presidential palace.

"We will soon organise a conference in Kabul to organise new and effective ways to combat this problem," he said.

Karzai also invited his chief rival Abdullah Abdullah into work a government of national unity, and pledged action to fight drugs and create jobs.

"We have to learn from our mistakes and shortcomings of the last eight years," the 51-year-old Karzai said, wearing a traditional hat and colourful cape.

After eight years of war and instability, the West has been pushing Karzai to commit to concrete reforms to clean up his government and restore trust.

Clinton, in Kabul for the first time as secretary of state, said the nation faced a "critical moment".

"There is now a clear window of opportunity for President Karzai and his government to make a compact with the people of Afghanistan," she said on the eve of Thursday's ceremony.

Washington has increasingly expressed concerns about Karzai's reliability as a US ally and effective head of state, urging his government to eradicate corruption to counter an intensifying Taliban-led insurgency.

Clinton has directly linked future levels of military and financial aid -- on which impoverished and war-torn Afghanistan depends -- to progress in eradicating official corruption.

Yet the United States and NATO -- with 100,000 soldiers fighting the Taliban and leaders deciding whether to dispatch tens of thousands of extra troops in a last-ditch effort to win the war -- have little choice but to work with Karzai.

President Barack Obama has said his decision on strengthening the US deployment is close and that he was weeks away from unveiling a war strategy review, a decision made no easier by Afghanistan's disputed August election.

Obama's administration has warned Afghans that America's military commitment there, more than eight years after the 2001 US-led invasion toppled the Taliban regime and swept Karzai to power, will not be "open-ended".

Leading rights groups Amnesty International and Human Rights Watch called on Karzai to now sever his links with warlords and abusers of human rights.

Karzai should "prioritise human rights and the rule of law... to strengthen the country's stability and security," said Amnesty.

Scepticism about his willingness to comply with conditions for continued Western support will be difficult to dispel, particularly with Fahim accused of human rights abuses and drug trafficking, in his administration.

The Afghan capital was on high alert for Taliban attacks to coincide with the inauguration, with many foreign employees of embassies, the United Nations and aid groups ordered to remain indoors.

Armed police and paramilitary units patrolled most roads and intersections while army, police and intelligence threw a ring of steel around the city.

Few cars were on the mostly closed roads and pedestrians were being stopped at checkpoints set up for the day.

Kabul has been the scene of a series of massive suicide car bomb attacks that have killed around 100 people in the last three months alone.

To many Afghans, Karzai's presidency lacks legitimacy, his government lacks authority and the way in which he took the presidency lacks credibility.

"Karzai has to earn political capital because he has none left," one diplomat told AFP on condition of anonymity.

Karzai was declared re-elected on November 2 by his own officials after a UN-backed commission found nearly a third of votes cast for Karzai on August 20 were fraudulent and his challenger Abdullah Abdullah abandoned a run-off.

In 2004, Karzai won Afghanistan's first presidential election with 55.4 percent of the vote.