This photo, carried by North Korea's official Korean Central News Agency, Jan. 16, shows the North's leader Kim Jong-un attending a a key parliamentary meeting of the Central Committee of the ruling Workers' Party of Korea the previous day. Yonhap

The United States expressed disappointment over the escalation of North Korea's rhetoric against South Korea on Monday, stressing inter-Korean cooperation is vital to fostering lasting peace on the Korean Peninsula.

A State Department spokesperson made the remarks after North Korean leader Kim Jong-un called for revising the country's constitution to define the South as its "invariable principal enemy" at a key parliamentary meeting Monday (Korean time).

"We are disappointed by the DPRK's continued rejection of dialogue and the escalation of its hostile rhetoric towards the ROK," the spokesperson said in response to a question from Yonhap News Agency. "We believe inter-Korean cooperation is vital to achieving lasting peace on the Korean Peninsula."

ROK and DPRK stand for the official names of South and North Korea, the Republic of Korea and the Democratic People's Republic of Korea, respectively.

The official also reiterated that the U.S. harbors no hostile intent toward the North.

"We continue to consult closely with the Republic of Korea, Japan, and other allies and partners about how to best engage the DPRK, deter aggression and coordinate international responses to the DPRK's ongoing and repeated violations of U.N. Security Council resolutions," the official said.

In his parliamentary speech, the North Korean leader also made a call to revise the basic law to codify the commitment to "completely occupying" the South Korean territory in the event of war and to draw up legal measures to define South Korea not as a counterpart for reconciliation and unification. (Yonhap)