Kissinger, who served under Presidents Richard Nixon and Gerald Ford, met with Trump after he won the presidency and earlier during his campaign. 

Kissinger also met with China's Xi Jinping on Friday, which some saw as an attempt to ease concerns there about the incoming Trump administration. 

The meeting came as Trump took a phone call with Taiwanese President Tsai Ing-wen, a break with decades of U.S. foreign policy that could cause friction between the U.S. and China. 

On Monday, Trump also met with Robert McFarland, who served as President Ronald Reagan's national security adviser from 1983 to 1985.