U.S. President Barack Obama says the days when North Korea could create a crisis and elicit concessions are over.

Obama made the remark when he held a joint news conference with President Park Geun-hye after the Seoul-Washington summit in Washington on Tuesday.

He said South Korea and the U.S. “are prepared to engage with North Korea diplomatically and, over time, build trust.” However, he added that “the burden is on Pyongyang to take meaningful steps to abide by its commitments and obligations, particularly the denuclearization of the Korean Peninsula.”

Obama also said that if the North thought its recent threats would drive a wedge between South Korea and the United States, today is further evidence that North Korea has failed again.

He then stressed that the United States and South Korea “are as united as ever” while the North, faced with new international sanctions, “is more isolated than ever.”