North Korea fired more than two dozen short-range missiles from its east coast into the sea last weekend. South Korean government spokesman Kim Min-seok told CNN the launches were "provocative."
Long-range missile tests from North Korea have preceded nuclear tests in the past. Researchers at Johns Hopkins University said on their Blog 38 website there was construction activity observed recently at the Punggye-ri nuclear test site, the Sohae satellite launching station and the Tonghae satellite launch site.
"While there are no signs of impending tests, activities at the three facilities indicate that Pyongyang is increasing its ability to conduct future tests," the researchers said Thursday.
The U.N. Security Council in early March said it determined "that proliferation of nuclear, chemical, and biological weapons, as well as their means of delivery, continues to constitute a threat to international peace and security."
The Security Council imposed tough sanctions on North Korea in response to nuclear tests in 2006 and 2009. Sanctions were tightened further in response to a January 2013 missile test and again in response to a March 2013 nuclear test.