Burns traveled to Kyiv earlier this month, CNN said. His trips are usually kept secret, The New York Times reported.
"While there, he reinforced the US commitment to provide support to Ukraine in its fight against Russian aggression, including continued intelligence sharing," said on official to CNN, speaking on the condition of anonymity.
It is unclear how many trips Burns has made to Ukraine since the start of Russia's invasion in February. However, he also flew to Kyiv immediately before the invasion, according to The Times.
The report came after US officials have become increasingly worried that President Vladimir Putin could use a "dirty bomb" to escalate the war as Russia continues to suffer setbacks in Ukraine.
A "dirty bomb" is a type of explosive that uses conventional explosives to spread radioactive material.
But Burns told CBS News last month that the intelligence community has not seen "any practical evidence" that there is an "imminent threat."
US intelligence has extensively helped Ukraine oppose Russia, including in its counteroffensive in the northeastern Kharkiv region last month, The Hill reported.
