U.S. President Barack Obama has extended executive orders imposing sanctions on North Korea, saying the communist nation still constitutes an "unusual and extraordinary threat" to the U.S.

In a letter sent Monday, Obama notified Congress of the continuation of North Korea's designation as a "national emergency," which serves as the basis for a series of executive orders calling for sanctions on Pyongyang for provocations, such as the 2010 sinking of a South Korean warship.

By law, such national emergencies should be extended every year.

As reasons for the continuation, Obama cited the North's proliferation of "weapons-usable fissile material" on the Korean Peninsula, its actions and policies that destabilize the peninsula and imperil U.S. forces, allies, and trading partners in the region, and other "provocative, destabilizing, and repressive actions and policies."

The North continues to "constitute an unusual and extraordinary threat to the national security, foreign policy, and economy of the United States. For this reason, I have determined that it is necessary to continue the national emergency with respect to North Korea," Obama said in the letter.

The measure extended Executive Orders 13466, 13551, 13570 and 13687 that were issued in 2008, 2010, 2011 and this year, respectively. Executive Order 13551 was designed to punish the North for sinking the South Korean warship Cheonan and Executive Order 13687 came in response to the North's cyber-attack on Sony Pictures. (Yonhap)