WASHINGTON—U.S. intelligence officials have met with North Korean counterparts secretly for a decade, a covert channel that allowed communications during tense times, aided in the release of detainees and helped pave the way for President Trump’s historic summit last year with North Korean leader Kim Jong Un.

The secret channel between the Central Intelligence Agency and spies from America’s bitter adversary included two missions to Pyongyang in 2012 during the Obama administration by Michael Morell, then deputy CIA director,...