"If we are to succeed, North Korea must set aside its search for obstacles to negotiations and instead seek the opportunities for engagement while that opportunity lasts," Stephen Biegun said Friday in a speech at the University of Michigan.

The State Department released the text of his remarks on Saturday.

The two countries began a historic dialogue after President Donald Trump and North Korean leader Kim Jong Un held a summit in Singapore in June 2018.

A second summit in Hanoi in February collapsed without an agreement. The pair met again in June in the Demilitarized Zone dividing North and South Korea, and agreed there to restart working-level dialogue, but those talks have yet to begin.

In recent weeks North Korea has carried out a series of tests of short-range missiles. US officials have called these launches provocations, although Trump himself has avoided criticizing them.

North Korean officials have also criticized the US position that sanctions against North Korea will not be lifted until the country gives up its nuclear weapons.

"We have made clear to North Korea that we are prepared to engage as soon as we hear from them. We are ready, but we cannot do this by ourselves," Biegun said in his speech.

"We must set in motion an intensive set of negotiations," he added.

He also raised the prospect of "immediate actions" that might be taken if the nuclear talks make progress in moving away from hostility and distrust.

"It is clear that both sides can quickly agree to significant actions that will declare to our respective peoples -- and to the world -- that US-North Korea relations have taken an irreversible turn away from conflict," Biegun said, without giving an idea of what such actions might be.

He once again criticized the idea of a phased approach as advocated by some experts and sought by North Korea, which wants Washington to ease sanctions in exchange for the North taking steps toward denuclearization.