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Anjum Naveed, Associated Press
Pakistan's President Asif Ali Zardari, widower of assassinated former Pakistani leader Benazir Bhutto gestures during a press conference in Islamabad Tuesday.

ISLAMABAD, Pakistan — Asif Ali Zardari, the widower of assassinated former Pakistani leader Benazir Bhutto, began his term as Pakistan's new president Tuesday by declaring the war on terrorism a chance for the country to overcome its economic problems.

Three days after winning election by legislators, Zardari took the oath of office at a brief ceremony in the presidential palace vacated by Pervez Musharraf, who resigned last month under pressure.

With his three children and Bhutto's sobbing sister among the well-wishers and dignitaries packing a cavernous hall, Zardari beamed as the ceremony ended amid shouts of "Bhutto is alive!"

But in the front row sat a reminder of his task ahead: Afghan President Hamid Karzai, whose government has accused Pakistan of failing to take action against Taliban militants based around the countries' common border.

Karzai, the only foreign head of state to attend, said afterward that Pakistan's new government augured well for both countries and said Zardari's "intentions" echoed his own.

"For each step that you take in the war against terrorism for bringing peace to two countries, for bringing stability to two countries, Afghanistan will take many, many steps with you," he said.

Asked about allegations that Pakistan's intelligence agency had collaborated with militants waging war on Afghanistan's U.S.-backed government, Zardari said the two neighbors would work together on any "weaknesses."

President Bush said he called Zardari on Tuesday to pledge his administration's full support to Pakistan as it fights terrorists and extremists in its border regions with Afghanistan.

"Defeating these terrorists and extremists is in Pakistan's interest," Bush said in a speech at the National Defense University. "They pose a mortal threat to Pakistan's future as a free and democratic nation. Defeating these terrorists and extremists is also Pakistan's responsibility because every nation has an obligation to govern its own territory and make certain that it does not become a safe haven for terror."

Bush said the United States and its NATO allies will continue helping Pakistan in its efforts to defeat extremists.

"The same terrorists who murdered innocent civilians in Karachi and Islamabad are also plotting new attacks against the United States and Europe," he said.

Zardari made clear that his government had no intention of ending its ties with the United States, which has poured billions of dollars into Pakistan in return for cooperation.

Those funds have helped stabilize the Islamic world's only nuclear power by generating economic growth. However, many Pakistanis oppose their country's role in the war on terror, and blame it for fanning religious extremism.

"As far as America is concerned, the fact that we are on the globe and we are in the eye of the storm, I consider that an opportunity," Zardari said at a joint news conference, noting that most countries welcome foreign investment.

"I intend to take that and make it our strength. We intend to take the world with us in developing the future of Pakistan and changing the future of our neighbors also," he said.

The inauguration of Zardari, 53, completes Pakistan's return to civilian rule nearly nine years after then-army chief Musharraf seized power in a bloodless military coup.

The United States came to depend heavily on Musharraf for cooperation to capture or kill al-Qaida leaders who plotted the 9/11 attacks on America and fled Afghanistan after the U.S.-led invasion in 2001 toppled their Taliban allies.

However, the Taliban revived on Musharraf's watch, and al-Qaida chiefs Osama bin Laden and Ayman al-Zawahri remain on the run, probably somewhere in the uncontrolled frontier region.

On Tuesday, Zardari cited the assassination of his wife in a gun-and-bomb attack on the campaign trail in December to bolster his anti-terrorist credentials. He also said a democratic Pakistan would "stand with" all its neighbors, which include archrival India.

In a demonstration of Pakistan's resolve, its army says it has killed hundreds of rebels in ongoing operations in several parts of Pakistan's volatile northwest.

The Pakistani Taliban have responded with suicide bombings, including one in the city of Peshawar that killed 35 people Saturday, the same day as the presidential election by lawmakers.

Meanwhile, a rare assault by U.S. ground troops and a series of missile strikes into Pakistan's tribal region indicate that Washington is getting more aggressive about militant havens just beyond the Afghan border, despite intensifying Pakistani protests.

Officials said Tuesday the death toll from the latest missile strike had risen to 20 after residents and militants pulled more bodies from the rubble of a seminary and houses in a village in the North Waziristan region. Two Pakistani intelligence officials said the total included four suspected foreign militants.

Musharraf quit reluctantly on Aug. 18 to avoid the threat of impeachment at the hands of a coalition of parties that routed his supporters in February parliamentary elections. He made himself deeply unpopular by imposing emergency rule and purging the Supreme Court in November in order to halt legal challenges to his continued rule.