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By Caren Bohan
SHANGHAI (Reuters) - President Barack Obama said on Monday that al Qaeda remained the biggest threat to U.S. security, as his aides stepped up pressure on Afghanistan and Pakistan to cooperate with Washington's strategy in the troubled region.
In a sign the pressure was working, Afghanistan later announced it was forming a new anti-corruption unit to fight rampant graft, seen as critical in winning back the people's support in the war against a resurgent Taliban.
Obama, who was visiting Shanghai as part of a nine-day Asian tour, is nearing a decision on whether to send up to 40,000 more troops to fight the eight-year-old war in Afghanistan.
He has faced criticism at home for "dithering" on the Afghan war strategy, and the political pressure has been rising to make a decision soon.
"I continue to believe that the greatest threat to the United States' security are the terrorist networks like al Qaeda," Obama told Chinese students at a town hall meeting in Shanghai.
"They have now moved over the border of Afghanistan and are in Pakistan, but they continue to have networks with other extremist organizations in that region and I do believe it is important for us to stabilise Afghanistan."
One of Obama's top aides had delivered a letter to Pakistani President Asif Ali Zardari urging him to work with the United States against extremists, the New York Times reported on Monday.
In the letter, Obama offered Zardari a range of new incentives to the Pakistanis for their cooperation, including enhanced intelligence sharing and military cooperation, according to the Times, which said the missive was delivered in person by National Security Adviser General James Jones.
Jones had travelled to Pakistan just before joining Obama over the weekend in Singapore for a summit of Asia Pacific leaders.
In addition to Zardari, Jones met with Pakistani Prime Minister Yusuf Raza Gilani, Army chief General Ashfaq Kayani and other officials.
Jones's press secretary, Mike Hammer, would not discuss what was said in the meetings nor whether a letter was delivered.
In Islamabad, Pakistani Foreign Ministry spokesman Abdul Basit confirmed Jones had delivered a letter but declined to give details.
"It was a diplomatic communication," said Basit, who also declined to comment on the reported U.S. call for Pakistan to do more.
The effort to seek greater cooperation from Pakistan came as Secretary of State Hillary Clinton called on Afghan President Hamid Karzai to do a better job of tackling corruption within his government and create a major crimes tribunal.
"We're going to be doing what we can to create an atmosphere in which the blood and treasure that the United States has committed to Afghanistan can be justified and can produce the kind of results that we're looking for," Clinton told ABC News.
"Now, we believe that President Karzai and his government can do better. We've delivered that message," she added.
On Monday, Afghanistan's Interior Ministry said it would form a high-level anti-corruption unit to investigate and prosecute graft among senior officials.
"President Hamid Karzai, after being re-elected for another five years, has dedicated his five years to fighting corruption," Interior Minister Hanif Atmar told a news conference.
Atmar was flanked by U.S. ambassador Karl Eikenberry and British ambassador Mark Sedwill as he made the announcement.
"(Corruption) requires action. Words are cheap. Deeds are required," Eikenberry told the news conference.
Obama is facing dissent among his advisers over the strategy in Afghanistan.
General Stanley McChrystal, the top U.S. and NATO commander in Afghanistan, has requested 40,000 more troops for the war and says the mission is at risk of failure without them.
Last week, it emerged that Eikenberry had expressed deep concerns in memos to the president about sending in more troops until Karzai's government improved its performance.
Criticism of the delays in the decision-making process have increased lately.
"It is evident from the length of this deliberative process and from the flood of leaks that have emerged from Kabul and Washington that the perfect course of action does not exist," David Broder, a prominent columnist wrote in the Washington Post.
"Given that reality, the urgent necessity is to make a decision -- whether or not it is right," Broder said.