WASHINGTON -- The United States is disappointed in North Korea's recent actions to sever ties with South Korea, the State Department said Tuesday, urging the communist nation to return to diplomacy and cooperation.

The comments came in response to a series of actions North Korea has taken in anger over anti-Pyongyang leaflets sent by defectors into the North.

Earlier Tuesday, the North vowed to cut off all inter-Korean communication lines and refused to answer South Korea's phone calls via liaison and military hotlines.

"The United States has always supported progress in inter-Korean relations, and we are disappointed in the DPRK's recent actions," a department spokesperson told Yonhap News Agency, referring to North Korea by its official name, the Democratic People's Republic of Korea.

"We urge the DPRK to return to diplomacy and cooperation," the spokesperson continued. "We remain in close coordination with our ally, the Republic of Korea, on efforts to engage the DPRK."

The statement could be seen as a public call on North Korea to cease its threats and de-escalate tensions with the South.

In announcing that it would cut off all communication lines at noon Tuesday, North Korea said it will now treat South Korea as an "enemy."

Before that announcement, the State Department had not specifically addressed the North's threats that began last week, repeating its position that the US supports inter-Korean cooperation and coordinates with South Korea to ensure cooperation "proceeds in lockstep with progress on denuclearization."

Inter-Korean relations have deteriorated as nuclear talks between Washington and Pyongyang have ground to a halt due to differences over the scope of North Korea's denuclearization and sanctions relief from the United States.

South Korea urged the North to maintain the communication lines, citing inter-Korean agreements.

The unification ministry, which handles inter-Korean relations, said the government will continue to "make efforts for peace and prosperity on the Korean Peninsula."

The leaflets have long annoyed Pyongyang because they contain messages sharply critical of the regime and its leader, Kim Jong-un, along with one-dollar bills and USB memory sticks meant to encourage North Koreans to pick them up.

The South Korean government, which has previously said the activists are exercising their right to freedom of speech, is moving to legislate a ban on sending the leaflets. (Yonhap)