US President Barack Obama said he told Chinese President Xi Jinping that Beijing should work harder to change Pyongyang's behavior if it doesn't want to see the THAAD missile defense system deployed to South Korea.

"China continues to object to the THAAD deployment in the Republic of Korea, one of our treaty allies. And what I've said to President Xi directly is that we cannot have a situation where we're unable to defend either ourselves or our treaty allies against increasingly provocative behavior and escalating capabilities by the North Koreans," Obama said at a news conference in Laos after the East Asia Summit.

"And I indicated to him that if the THAAD bothered him, particularly since it has no purpose other than defensive and does not change the strategic balance between the United States and China, that they need to work with us more effectively to change Pyongyang's behavior," he said, according to a White House transcript.

Obama and Xi held one-on-one summit talks on the sidelines of the G-20 summit in China last week.

Obama said that China has done more to carry out the latest UN sanctions on the North than before, but there are "still places where they need to tighten up".

"We continue to indicate to them the importance of tightening those up," he said.

Obama acknowledged that his policy on North Korea has failed to change the North's behavior.

"When it comes to changing Pyongyang's behavior, it's tough," he said.

Should the North be serious about denuclearization talks, the US is ready to have such talks.

"It's not as if we are looking for a problem, or avoiding a willingness to engage diplomatically. But diplomacy requires that Pyongyang meet its international obligations," he said.

"Not only is it failing to meet those international obligations, it's not even suggesting that they have any intention to do so anytime in the future regardless of the inducements that might be put on the table," he added.

Obama said the US is "deeply disturbed by what's happened," pledging to make sure to defend itself and allies while at the same time ensuring to "put some of the toughest pressure that North Korea has ever been under as a consequence of this behavior." (Yonhap)