North Korean leader Kim Jong Un called on his scientists to do more to boost the country's arsenal after the regime conducted its fourth nuclear test last week in defiance of neighbors including China and South Korea.

Speaking to the scientists responsible for building what North Korea said was a hydrogen bomb, Kim said it was his "expectation and conviction" they would advance their research, the official Korean Central News Agency said Monday. Kim has said the nuclear program is defensive and designed to prevent a U.S.-led invasion.

Kim's comments come as the U.S. and South Korea seek international support to punish the isolated nation for the test and pressure it to halt its nuclear program. The U.S. on Sunday sent a B-52 bomber on a low-level flight south of Seoul, while South Korea has restarted propaganda broadcasts across its heavily armed northern border. The loudspeaker bursts risk pushing South Korea and North Korea "toward the brink of war," North Korean official Kim Ki Nam said on Friday, Yonhap reported.