New York City Mayor Bill de Blasio emerged from the depths of the nation's largest transit system Thursday to assure commuters that trains are safe in the wake of reports of an ISIS terrorist plot against U.S. subways.

"I have a simple message for all New Yorkers: There is no immediate credible threat to our subway system," de Blasio told reporters after taking a short subway ride from City Hall to Union Square in Manhattan. "I say that with confidence. People should go about their business as they normally would."

Intelligence and law enforcement agencies have no indications of an ISIS terror plot against U.S. transit systems, two U.S. law enforcement officials told CNN earlier.

Later on Thursday, Deputy Assistant Secretary of State for Iran and Iraq Brett McGurk told CNN's Brianna Keilar that there "is no specific credible threat whatsoever that they have uncovered to the United States." He spoke to Keilar having just come from a meeting with Iraqi Prime Minister Haider al-Abadi and Vice President Joe Biden.

From Washington to New York, a flurry of denials followed media reports that Iraqi Prime Minister Haider al-Abadi told reporters at the United Nations that his country's intelligence agency had uncovered an imminent ISIS plot against United States and Paris subways.

"We don't have anything to back it up at this point," State Department spokeswoman Marie Harf told CNN. "We'll keep looking at it."

A senior administration official told CNN that "no one in the U.S. government is aware of such a plot and it was not raised with us in our meetings with Iraqi officials" at the United Nations, including a meeting between al-Abadi, President Barack Obama and Secretary of State John Kerry.

FBI Director James Comey told reporters that he hadn't heard of the plot.

U.S. agencies are reaching out to Iraq to determine what information it may have, the two officials told CNN.

"We know that our transit and aviation systems are always a target. We know New York is always a target," one U.S. law enforcement official told CNN.

"Do we know what the Iraqi Prime Minister is talking about? No," the official said.

(CNN)