U.S. President Barack Obama has welcomed South Korea's decision to fully participate in the U.S.-led Proliferation Security Initiative.

The White House said Obama thanked President Lee Myung-bak for joining the PSI during a Tuesday phone conversation concerning North Korea's test of a nuclear device on Monday.

Obama also vowed Washington's continued support to assist in the defense of South Korea. The two leaders agreed to cooperate in backing a powerful resolution they expect the U.N. Security Council will soon adopt denouncing North Korea for the nuclear test.

The U.S. president has consistently stressed the need for strong and comprehensive global cooperation, through programs such as the PSI, in order to prevent the spread of weapons of mass destruction that threaten world security.