"If our leaders seek to conceal the truth, or we as people become accepting of alternative realities that are no longer grounded in facts, then we as American citizens are on a pathway to relinquishing our freedom," Tillerson said in a rare public address to graduates at the Virginia Military Institute.

It is the "responsibility of every American citizen to protect our freedom by recognizing what truth is and is not," Tillerson said.

"When we as people go wobbly on the truth, even on what may seem as trivial matters, we go wobbly on America.

"If we do not as Americans confront the crisis of ethics and integrity in our society and among our leaders then American democracy as we know it is entering its twilight years," he added.

Trump fired Tillerson in March, citing a "chemistry" problem between them, bringing to an end an acrimonious 13-month relationship. The 66-year-old's tenure was marked with a number of clashes over Trump's policies.

After Tillerson was quoted saying he had attempted to open dialogue with North Korea, Trump said on Twitter he told Tillerson he is "wasting his time trying to negotiate with Little Rocket Man," a reference to North Korean leader Kim Jong-un.

Tillerson, a Texan and the chief executive of energy company ExxonMobil for a decade before joining Trump's administration, was replaced with CIA directer Mike Pompeo who has since made two trips to North Korea to hold discussions with Kim.

Australian Associated Press