UNITED NATIONS - President Bush challenged the United Nations yesterday to live up to its ideals by fighting terrorism and promoting freedom around the world, while taking more vigorous action to end the killing in Sudan's Darfur region and pressure Myanmar's government to lift its repression.

Amid a gloomy atmosphere brought on by the world financial crisis, Bush focused his final address to the General Assembly on some of his favorite themes, especially the responsibility of member nations to battle terrorists, while seeking to reassure world leaders that the United States is taking steps to put its economic house in order.

He also upbraided Russia, calling its invasion of Georgia a "violation" of the U.N. Charter that sets forth the "equal rights of nations large and small."

Bush's administration has had a testy relationship with the United Nations, particularly over the widespread belief here that he took the United States to war in Iraq without a proper international mandate. Yesterday, Bush cited the U.N.'s extraordinary "potential" to solve world problems, but said it must be more transparent, accountable and vigorous in fighting terrorism and ending tyranny.

"In the decades ahead, the United Nations and other multilateral organizations must continually confront terror," he said. "This mission requires clarity of vision. We must see the terrorists for what they are: ruthless extremists who exploit the desperate, subvert the tenets of a great religion, and seek to impose their will on as many people as possible."

Bush said the United Nations must challenge tyrannical governments just as forcefully. "History shows that when citizens have a voice in choosing their own leaders, they are less likely to search for meaning in radical ideologies," he said.

Bush urged the United Nations to reform its Human Rights Council, which he complained has frequently protected violators. He called for a more determined effort to help the Burmese people free themselves from repression. And he said the Security Council needed to press Sudan to fulfill commitments to halt violence in Darfur, where a full complement of U.N. peacekeepers has yet to arrive, two years after their deployment was authorized here.

For the last eight years, he noted, the United Nations has had "successes and setbacks," and he added: "The United Nations and other multilateral organizations are needed more urgently" than ever. Bush, who seemed somewhat subdued in delivering his address, received a friendly, if not overwhelming, response from the delegates seated in the cavernous General Assembly chamber.

Later in the day, Iranian President Mahmoud Ahmadinejad sharply attacked the United States and NATO, accusing them of acting as aggressors in Iraq and Afghanistan, and of starting those wars "in order to win votes in elections."

"American empire in the world is reaching the end of its road, and its next rulers must limit their interference to their own borders," he said.

He reiterated Iran's insistence that its nuclear program was purely peaceful, not aimed at producing nuclear weapons as the Unietd States and some European countries suspect. "A few bullying powers have sought to put hurdles in the way of the peaceful nuclear activities of the Iranian nation," he said.

In interviews before his U.N. speech, Ahmadinejad blamed U.S. military interventions around the world in part for the collapse of global financial markets. "The world economy can no longer tolerate the budgetary deficit and the financial pressures occurring from markets here in the United States."

Bush spent the day in meetings with dignitaries gathered here for the opening session of the General Assembly. He met with the new Pakistani president, Asif Ali Zardari, the widower of slain politician Benazir Bhutto, a session that focused on the fight against Islamic radicals in the border region along the Pakistan-Afghanistan border.

Tensions have risen in recent weeks over U.S. concerns that Pakistan is not waging a strong-enough battle and Pakistani anger over U.S. incursions on its sovereignty.

Last night, Pakistani intelligence officers said soldiers shot down a suspected U.S. military drone close to the Afghan border. A senior U.S. official disputed the claim.

If verified, it apparently would be the first time a pilotless aircraft was brought down over Pakistan and would add to tensions over U.S. incursions into the country's lawless tribal regions.