International sanctions against North Korea's nuclear tests levied in the 2000s have been less effective due mainly to improved political and economic situation in the reclusive country, a report said Thursday.

In 1994, North Korea made its announcement to leave the Nuclear Non-Proliferation Treaty right after the U.S.-led sanctions to suspend development aid and impose an arms embargo were released.

But in the 2000s, North Korea carried out four nuclear tests, even after the United Nations came up with harsher punishments including financial sanctions, travel bans and arms embargoes.

"The effectiveness of international sanctions against North Korea has reduced since the mid-2000s, when North Korea's political and economic situation improved to some extent compared to the 1990s," said the report by state-run think tank Korea Development Institute (KDI).  

"The political situation in the 2000s is steadier than that of the 1990s," it said.    

The KDI report also noted that North Korea has become more confident in dealing with its internal politics and economic policies during the past 10 years.

The latest nuclear tests conducted in early 2016 also brought about the strongest-ever sanctions led by the United States, including a "secondary boycott," referring to penalties on third countries' individuals and organizations who do business with North Korea.    

North Korean leader Kim Jong-il and his son Kim Jong-un have cemented their political grip through a constitutional revision and a number of political purges, becoming strong enough to resist the international sanctions and keep carrying out nuclear tests.

At the same time, the North Korean economy has posted positive growth since 1999, snapping a decade-long recession stemming from a series of flood and drought.

"Stronger sanctions and tighter implementation will have an effect on North Korea," said the KDI report. (Yonhap)