China has strengthened security checks for flights heading to the United States after a Nigerian man attempted to detonate explosives aboard a Northwest Airlines flight to Detroit on Christmas day.

A secondary security check is required at the boarding gates for America-bound flights, passengers have been told to report at airports earlier, and are not allowed to take any liquid in their carry-on luggage.

"We received instructions from the Civil Aviation Administration of China to go through additional procedures before the final boarding," said a spokeswoman for Shanghai Airport Authority surnamed Shen.

The measures were taken in response to an emergency plea issued by the US Department of Homeland Security seeking additional security checks for all US-bound flights and US airlines.

Shanghai Airport Authority operates the city's two international airports - Shanghai Pudong International Airport and Hongqiao Airport. Security checks for flights to other destinations are unchanged, she said.

China Eastern Airlines has forbidden passengers from carrying any liquid with them on flights to the US. However, they are allowed to take liquid containers of less than 100 ml in carry-on luggage on other international routes.

"We recommend those traveling to the US arrive at the airport two-and-half hours before departure to allow sufficient time for security checks," said Tom Xu at United Airlines' Shanghai Office. The time used to be two hours earlier.

The increased checks have not caused congestion at airports, Shen said. There is no decrease in the number of flights to the US, according to the Airport Command Office of Pudong Airport.

Similar measures are in place in Beijing Capital International Airport and Guangzhou Baiyun Airport, where passengers have to arrive at least three hours in advance.

In addition, an hour before the flight lands, passengers must not leave their seats, cannot read newspapers or use blankets and electronic items.