North Korea has challenged United Nations chief Ban Ki-moon to clarify the legal basis for UN sanctions against it, Pyongyang's state media said Tuesday.

The North's permanent representative to the UN wrote to Secretary-General Ban on Monday, saying that UN sanctions imposed over its nuclear tests and "peaceful satellite launches" had no legal foundation, KCNA news agency reported.

It did not identify the permanent representative by his name, Ja Song-Nam.

The UN Security Council in March imposed its toughest sanctions on North Korea to date following its fourth nuclear test in January and a long-range rocket launch the following month.

The North had already been subject to a range of sanctions because of earlier nuclear tests -- in 2006, 2009 and 2013 -- and a series of long-range missile tests presented as satellite launches.

The Security Council resolutions condemned the nuclear tests and rocket launches as a threat to international peace and security.

But the North Korean envoy asserted that no international laws or agreements -- such as the UN charter and the nuclear Non-Proliferation Treaty -- stipulate that nuclear tests are a threat to international peace and security.

The second question, he said in the letter, was why the Security Council had never sanctioned the United States or other countries for their nuclear tests or ballistic missile launches.

If no "convincing legal clarification" was given on such questions, the Security Council would be seen as enforcing double standards, the envoy said.

The North insists it needs atomic weapons to defend itself from what it calls a US nuclear threat.

A rare ruling party congress this month depicted the North as a fully-fledged nuclear weapons state and endorsed a push to improve and expand the arsenal.