London, Nov 1 (IANS): US Secretary of State John Kerry has said that, in some cases, the surveillance by the US National Security Agency (NSA) "reached too far inappropriately".

Kerry said Thursday he would work with President Barack Obama to make sure that this does not happen in the future, reported BBC.

The secretary also defended the NSA's acts, saying it had prevented terrorists attacks.

"We have actually prevented airplanes from going down, buildings from being blown up, and people from being assassinated because we've been able to learn ahead of time of the plans," Kerry was quoted by BBC as telling a conference in London via video link.

"I assure you, innocent people are not being abused in this process, but there's an effort to try to gather information. And yes, in some cases, it has reached too far inappropriately," Kerry said.

"And the president, our president, is determined to try to clarify and make clear for people, and is now doing a thorough review in order that nobody will have the sense of abuse... we are going to make sure that does not happen in the future," he added.

In recent weeks, the US government has struggled to quell a new diplomatic storm over reports that the American agency had monitored the cellphone of German Chancellor Angela Merkel for more than a decade. Brazil, France and Spain also voiced their outrage over US spying on their leaders.

China Thursday expressed grave concern over reported phone tapping by the US in China and sought an explanation from Washington.

"We have made representations to the US side, and we demand that the US side clarify and explain itself on this matter," Xinhua quoted Chinese foreign ministry spokeswoman Hua Chunying as saying in a press briefing.