U.S. President Joe Biden on Monday threatened to reimpose sanctions on Myanmar following a coup by the country's military leaders and called for a concerted international response to press them to relinquish power as the whereabouts of detained elected leader Aung San Suu Kyi remained unknown more than 24 hours after her arrest.

Biden condemned the military's takeover from the civilian-led government on Monday and its detention of Nobel laureate Suu Kyi as "a direct assault on the country's transition to democracy and the rule of law."

The crisis in Myanmar, also known as Burma, marks a first major test of Biden's pledge to collaborate more with allies on international challenges, especially on China's rising influence. That stance contrasts with former President Donald Trump's often go-it-alone "America First" approach.