The U.S. State Department said on Tuesday that the United States is "disappointed" in North Korea's recent actions, after the North cut off all communication lines with South Korea. 

A spokesperson for the department gave the response to an inquiry by Seoul-based Yonhap News, saying that the U.S. has always supported progress in inter-Korean relations. 

The spokesperson said that the U.S. urges North Korea to return to diplomacy and cooperation, adding Washington remains in close coordination with its ally, South Korea, on efforts to engage the North. 

It is unusual for the department to use "disappointed" in its official comments on North Korea issues. 

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