North Korea has reportedly rejected a formal timeline for its denuclearization proposed by Secretary of State Mike Pompeo.

Vox reported Wednesday that Pyongyang has rejected the timeline several times over the past two months amid continued negotiations over North Korea's nuclear program.

The timeline Pompeo proposed would mandate North Korea hand over 60 to 70 percent of its nuclear warheads to a third party within six to eight months, according to the report.

However, it is unclear how many warheads North Korean leader Kim Jong Un has, making it difficult to verify that Pyongyang has actually turned over an agreed-upon percentage.

One source told Vox that Pompeo's main priority at this stage in negotiations is to nail down the total number of nuclear weapons that North Korea officially possesses.

The State Department didn't immediately comment for the Vox report.

Trump administration officials in recent weeks have expressed frustration with North Korea's efforts to denuclearize despite President Trump hailing his June summit with Kim in Singapore as a success.

While Trump has celebrated North Korea's release of three American prisoners earlier this year and the return of what North Korea says are U.S. remains from the Korean War, Pyongyang has continued to rebuff U.S. efforts to give up its nuclear arsenal.

"The ultimate timeline for denuclearization will be set by Chairman Kim, at least in part," Pompeo told Channel NewsAsia in an interview last week.

"The decision is his. He made a commitment, and we're very hopeful that over the coming weeks and months we can make substantial progress towards that and put the North Korean people on a trajectory towards a brighter future very quickly."

White House national security adviser John Bolton told Fox News on Tuesday that "North Korea that has not taken the steps we feel are necessary to denuclearize."