Obama says al Qaeda still greatest threat to U.S.

* Aides push for greater Afghan, Pakistan cooperation

* Obama faces rising pressure to decide Afghanistan plan

* Top U.S. adviser delivers letter to Zardari -- NY Times

By Caren Bohan

SHANGHAI, Nov 16 (Reuters) - U.S. President Barack Obama on Monday called al Qaeda 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.

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.[ID:nN15457317]

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's press secretary, Mike Hammer, confirmed that 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.

But Hammer would not discuss what was said in the meetings nor whether a letter was delivered.

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.[ID:nN15457785]

"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.

DISSENT

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 Obama's own ambassador to Kabul, former military commander Karl Eikenberry, had expressed deep concerns in memos to the president about sending in more troops until Karzai's government improved its performance.[ID:nN12421444]

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 on Sunday.

"Given that reality, the urgent necessity is to make a decision -- whether or not it is right," Broder said.

Obama did not discuss the timing of his decision-making at the Shanghai town hall but said ordering U.S. soldiers to battle was a "very difficult thing" and was one of the hardest parts of his job.

He said that a key priority is training Afghan security forces so that the United States can "begin to pull our troops out" without risking a return to the instability that allowed the Taliban to come to power.

(For more stories on the conflict in Afghanistan and Pakistan click [ID:nAFPAK]

(Editing by Sanjeev Miglani;Additional reporting by Chris Buckley in Beijing)