U.S. Secretary of Defense Mark Esper urged U.S. allies to increase investment in defense on Tuesday, saying that there can be no "free riders" in common security.

Esper made the call during a webinar hosted by the Washington-based think tank Atlantic Council, urging the U.S.' NATO allies to increase their contribution to collective security. 

The U.S. defense chief called on all U.S. allies to increase their defense spending to two percent of gross domestic product. 

Calling China and Russia the U.S.' primary competitors, Esper said Washington expects its allies to stand shoulder to shoulder in confronting China’s bad behavior and Russia’s aggression. 

He added that to overcome the increasingly complex threats in the 21st century and defend shared values, there can be no free riders in common security.

Esper also said the U.S. continues to face ongoing threats from rogue states such as North Korea and Iran.