US President Barack Obama said?in Shanghai on?Monday that the biggest threat to US security is the terrorist groups like al-Qaida.

"I do continue to believe the greatest threat to United States security are the terrorist networks like al-Qaida," Obama said during a dialogue with Chinese students in Shanghai.

He said US troops moved into Afghanistan because al-Qaida was being hosted by the Taliban in Afghanistan but the terror group has now moved over the border of Afghanistan and has networks with other extremist groups throughout the region.

The United States now has 68,000 troops fighting in Afghanistan. Obama is going to decide on whether to grant a request by the top US commander in Afghanistan, Stanley McChrystal, to send 40,000 more troops.

"I do believe it is important for us to stabilize Afghanistan, so that the people of Afghanistan can protect themselves, and they can also be a partner in reducing the power of those extremist networks," he said.

The US president admitted that defeating al-Qaida is a difficult task and is not just a military exercise.

Obama flew into Shanghai from Singapore on Sunday night to kick off his four-day visit to China, his first trip to the Asian country since taking office in January.

Later Monday, he will fly to Beijing, where he will hold talks with Chinese President Hu Jintao and meet with other Chinese leaders.