In December, the Trump administration reportedly requested Seoul's approval of Cha. At the time, sources told Yonhap the new envoy could take office before the PyeongChang Winter Olympics if the Senate quickly confirms his appointment.

The post has been vacant since the Trump administration came into office early last year, sparking concerns about close coordination between the allies at a time of heightened tensions over North Korea's nuclear weapons and ballistic missile programs.

If appointed, Cha would have become the second Korean-American to take the post after Ambassador Sung Kim, who served in the Barack Obama administration.

The withdrawal seems to signal Washington's intent to take a hard-line stance on both the nuclear issue and trade, according to a source in Washington who wished to speak on condition of anonymity. He added it will take at least another seven or eight months for a new person to be nominated.

In the interim, Marc Knapper, a civil servant, has served as charge d'affaires at the US Embassy in Seoul.

The Washington Post quoted a senior US government official as saying that although the administration has yet to nominate a new person, "It is our intention to do so as soon as we can find the appropriate candidate."

The paper also said "a flag was raised" during Cha's vetting process that made him unfit for the job, but it did not elaborate.

Still, it is unusual because Washington would not have sought Seoul's approval without completing the background check, according to the source. (Yonhap)