The US will not tolerate a nuclear-armed North Korea and will make utmost efforts to defend its allies from the North, Vice President Joe Biden said Friday, Seoul's Yonhap News Agency reported. "There should be no doubt that the US is committed to do what it takes to defend our allies and ourselves from North Korea's provocations," Biden aid in a speech at Yonsei University in Seoul. "The US will not accept and tolerate a nuclear-armed North Korea," the vice president said, warning North Korean leader Kim Jong-un that the country will not be rewarded for bad behavior. "The North should demonstrate its commitment to complete, verifiable and irreversible denuclearization," he also noted. Biden is in Seoul on the final leg of his three nation tour of Asia, which also took him to Japan and China. North Korea seems to be notably expanding activities at its main nuclear complex in Yongbyon, a US think tank said Thursday, based on analysis of new satellite imagery, according to Yonhap. The revelation adds to worries that Pyongyang may be turning back to a provocation cycle after months of unanswered peace overtures towards Washington. "Recent commercial satellite imagery shows that North Korea's Yongbyon nuclear center appears to be increasingly active. There are several signs of new and continued activity and progress in the construction of facilities," the Institute for Science and International Security said in a report. It cited commercial satellite photos taken Monday that show steam is present at the fuel fabrication complex. "The steam is emerging from a building that years ago was involved in converting yellowcake to natural uranium dioxide. One implication of the steam is that the building is operational as part of an effort to make fuel for the 5 MWe (megawatt electrical) reactor," the Washington-based institute said. Last month, the International Atomic Energy Agency announced it also detected the release of steam and water from the Yongbyon reactor in an indication that it has been restarted