Last week the Washington Post reported that Cha, who worked on the White House National Security Council in the George W. Bush administration, was dropped after he voiced objections to the Trump administration's so-called "bloody nose" strategy for delivering a targeted strike on North Korea.

The White House confirmed that Cha was no longer under consideration.

"The challenge posed by North Korea's nuclear and missile programs is perhaps the most significant foreign policy challenge our nation has faced in decades," the letter continued. "It is therefore shocking that the Administration -- a full year into its term -- has yet to formally nominate someone to be the ambassador, which is the highest-ranking US government official in South Korea."

The senators also underscored the risk of a limited strike escalating into full-scale war.

"Moreover, without congressional authorization a preventative or preemptive US military strike would lack either a Constitutional basis or legal authority," the letter said. (Yonhap)