SHANGHAI: President Barack Obama said on Monday that Al-Qaeda remained the biggest threat to US 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 Taleban.

“I continue to believe that the greatest threat to the United States’ security are 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 stabilize Afghanistan.” One of Obama’s top aides had delivered a letter to Pakistani President Asif Ali Zardari urging him to work with the US 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 Gen. James Jones. Jones had traveled 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 Yousaf Raza Gilani, army chief Gen. Ashfaq Kayani and other officials.

The effort to seek greater cooperation from Pakistan came as US 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.