"I'm confident that we can make sure that Pakistan's nuclear arsenal is secure. Primarily, initially, because the Pakistani army, I think, recognizes the hazards of those weapons falling in to the wrong hands," Obama said last week.

Zardari, who took the presidency in September 2008, acknowledged the existence of grave threat posed by the Islamic extremists. But he categorically rejected the saying that Pakistan's nuclear arsenal is not secure due to the weakness of his government.

"They are in safe hands...we have a 700,000 army -- how can they take over?" he heckled in an interview with the CNN on the eve of the tripartite summit.

Trilateral cooperation

Simultaneous visits by top leaders of Afghanistan and Pakistan to the White House are not something new.

On Sept. 28, 2006, former US President George W. Bush held trilateral talks with Karzai and then Pakistani President Pervez Musharraf, trying to appeal to the two bickering presidents to put aside their differences and "strategize together" on ways to defeat the common enemy of terrorism.

"We must work together with a renewed sense of partnership to share intelligence and to coordinate our efforts to isolate, target, and take out our common enemy," Obama said.

Pakistan has been one of the major allies of the Untied States in fighting against terrorism since the "9/11" attacks in 2001. However, concerns have been growing over the weakness of the Pakistani government amid the recent advances of Taliban militants in the country.

Obama also said he was gravely concerned over the "very fragile" Pakistani civilian government, an allegation Zardari has denied.

"They (Taliban militants) are not threats to my government...my government is not going to fall when one mountain is taken by one group or the other," he said.

As Washington began shifting its attention from Iraq to Afghanistan, it has no choice but to strengthen its support for Pakistan to tackle the increasingly difficult situation there, analysts say.

However, it remains a challenge for the three countries to better coordinate in fighting the Taliban insurgency.

Pakistan was furious about US-led cross-border airstrikes, which it deemed a violation of its sovereignty and often led to deaths of Pakistani civilians.

Meanwhile, the United States had blamed Pakistan for not doing enough to fight terrorists, an accusation denied by Pakistan.

US Secretary of State Clinton said on Wednesday she was impressed with recent efforts by Pakistan to fight the Taliban insurgency.

"I'm actually quite impressed by the actions that the Pakistani government is now taking," Clinton said, noting that there was a "resolve going forward" in Pakistan's struggle with the Taliban.

Only last month, Clinton warned that Pakistan was "abdicating" to the Taliban by allowing extremists to impose Islamic law in parts of the country.

(Xinhua News Agency May 7, 2009)