Former US president George W. Bush said on Saturday that the war against the Taliban in Afghanistan must be won to stop a return to "brutal tyranny" in the nation.

In a wide-ranging speech to a leadership conference in the Indian capital New Delhi, Bush said defeating the insurgents was "necessary for stability" and peace both in the region and globally.

"If the Taliban, Al-Qaeda and their extremist allies were allowed to take over Afghanistan again, they would have a safe haven and the Afghan people, particularly the Afghan women, would face a return to a brutal tyranny."

"This region and the world would face serious threats," he added.

Bush ordered US troops into Afghanistan, ousting the hardline Taliban regime in 2001 following the September 11 attacks on the United States.

But since then, Taliban resistance has grown increasingly violent.

He steered clear of advising President Barack Obama on future US military policy in the region but said that "the work is hard and I hope we don't abandon the people of Afghanistan".

A 'great friend'

His statements came as Obama was weighing whether to send thousands more Americans to war in Afghanistan.

Bush, on his second visit to India, was hosted at a lunch on Friday given by Prime Minister Manmohan Singh, who called him a "great friend" of the country.

During his period in the White House, Bush pushed laws through the US Congress ending India's nuclear pariah status and allowing the country access to civilian atomic technology.

Bush said both the United States and India were "involved in an ideological struggle against extremists who murder the innocent to advance a dark vision of extremism and control".

"They attack political, financial and diplomatic targets because they hate our way of life and they hate our vision for freedom and human rights and human dignity and prosperity and peace," Bush told the conference.

Obama not his 'first choice'

Bush was speaking less than a month before the one-year anniversary of the three-day Islamist militant assault on Mumbai which began on 26 November and left 166 people dead.

He left the White House last January with rock-bottom approval ratings and has made few public appearances since.

Bush said Obama was not his "first choice" for the White House "but I wish him every success and I'm not going to spend a lot of time criticising — he's got plenty of critics".