Signs that North Korea may be preparing for another nuclear test have once again led to feverish speculation what the North is or is not capable of.

The Defense Ministry here claimed Wednesday that the North appears to have improved its capacity to miniaturize nuclear warheads so they can be fitted on to missiles.

The nuclear payload needs to be reduced to less than 1,000 kg and the diameter to less than 90 cm to fit on a missile.

The ministry based its claim on assessment from South Korean and U.S. intelligence services.

"The North has reduced the nuclear payload to about 1,500 kg, but not less than 1,000 kg, which means that its nuclear weapons aren't warfare-ready yet," a ministry spokesman said. "But we presume that the North’s three previous nuclear tests have enabled it to improve technology to increase nuclear yield and make the payload smaller."

Meanwhile, experts said it seems unlikely that the North will conduct a nuclear test to coincide with U.S. President Barack Obama’s visit to Seoul this Friday and Saturday.

Based on "limited commercial satellite imagery," specialist website 38 North at Johns Hopkins University said recent operations at the test site in Punggye-ri, North Hamgyong Province "have not reached the high level of intensity -- in terms of vehicle, personnel and equipment movement -- that occurred in the weeks prior to past detonations. Moreover, other possible indicators present before the North Korean nuclear tests in 2009 and 2013… have not been spotted."

