A South Korean think tank estimates that North Korea needs to conduct at least four more nuclear tests before obtaining tactical nuclear weapons.

The Korea Institute for Defense Analyses presented the estimate in a brief submitted to a ruling party lawmaker on Monday. The institute is affiliated with the Defense Ministry.

The institute says North Korea is believed to have spent 1.1 billion to 1.6 billion dollars on nuclear development since 1970, including six nuclear tests.

It also says at least four additional tests are needed to develop tactical nuclear weapons that could be used for localized attacks and supersized nuclear warheads.

North Korea's ruling Workers' Party set the goal of developing such weapons at its convention in January last year.

The institute estimates that four additional tests would cost up to 640 million dollars.

The lawmaker who received the brief argued that the amount is enough to cover the cost of buying food that will be in short supply this year in North Korea.

Pyongyang adopted a law that stipulates conditions for the use of nuclear weapons at the Supreme People's Assembly on September 8.

North Korean leader Kim Jong Un told the assembly that the country will never abandon nuclear weapons.