Story highlights State Dept. official tells CNN "no specific credible threat whatsoever" uncovered in U.S.

U.S. intelligence, law officials say they have no indication of a transit system terror plot

Iraq's PM said his country uncovered a plot against U.S., Paris subways, reports say

New York Gov. Andrew Cuomo: State officials are treating the report with "utmost precaution"

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."

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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.

'We are monitoring these reports'

De Blasio stood with Police Commissioner William Bratton and FBI Assistant Director in Charge George Venizelos outside the Union Square subway hub for a news conference.

"We are convinced that New Yorkers are safe," he said. "We are convinced that people should go about their normal routine. Terrorists want us to live in fear."

As a precaution, the mayor said, New York will increase security measures throughout its vast transit network.

"You may be asked to open your bag," he said. "You may find there are some spot checkpoints set up. Don't be alarmed."

Bratton said New York was already on a high state of alert because of the world leaders in town for the U.N. General Assembly, including President Barack Obama.

In Connecticut, Governor Dannel Malloy ordered increased state police presence along Metro North and Amtrak rail lines, as well as at Bradley Airport.

New York Gov. Andrew Cuomo said state officials were treating the report with "utmost precaution" and "coordinating at a high level with local, state and federal partners."

"I want to assure the people of New York that we are monitoring these reports closely and are in close communication with officials in Washington," Cuomo said in a statement.

He said that New York and New Jersey authorities had in recent weeks increased security at mass transit sites and areas as part of a joint security enhancement.

John Miller, the New York Police Department's deputy commissioner for intelligence and counterterrorism, said in the statement that police were in "close contact" with the FBI and other federal agencies.

"New York City normally operates at a heightened level of security and we adjust that posture daily based on our evaluation of information as we receive it," he said.

The Metropolitan Transportation Authority, which operates the city's buses and trains, declined to comment. A spokesman for Transport Workers Union Local 100, which represents subway workers, said the MTA, as of Thursday afternoon, had not issued any security bulletins.

Earlier plots

European and New York transit systems have been targeted in the past.

On March 11, 2004, terrorists in Madrid, Spain, carried out coordinated bomb explosions on four rush-hour trains , killing 191 people and wounding more than 1,800.

The Madrid train bombings were the deadliest terror attack in Western Europe since the bombing of Pan Am Flight 103 over Lockerbie, Scotland, in 1988, which killed 270 people.

The terrorists in Madrid, the authorities later determined, carried bombs in sports bags and backpacks onto four commuter trains serving the east of the Spanish capital. They got off and left behind their deadly devices.

Spanish courts later convicted 14 Islamist militants for their roles in the bombings, along with four Spaniards for trafficking in explosives used in the attacks.

On July 7, 2005, 52 people were killed and 700 injured when four bombers planted explosives on three underground trains and a bus in London.

In 2005, 52 people were killed and 700 injured when bombers planted explosives in London.

The bombers, from the north of England, used cheap explosives and techniques found on the Internet to carry out the country's worst terror attack.

British intelligence came under criticism after reports that the country had received reliable warnings about such an attack.

Two weeks after the London transit bombings, an attempted second wave of bombings struck other trains and a bus, but the devices failed to explode properly. More than a dozen people were arrested afterward.

Four homemade bombs stuffed into backpacks did not fully explode in the attempt. One person was injured.

In 2012, a Bosnian immigrant accused of plotting to bomb New York's subway system as an "al Qaeda terrorist" was found guilty of conspiracy to use weapons of mass destruction, conspiracy to commit murder, supporting a foreign terrorist organization and other charges.

Prosecutors said Adis Medunjanin traveled to Pakistan's tribal region with two high school friends, Pakistani-born Najibullah Zazi and Zarein Ahmedzay, an immigrant from Afghanistan. His friends pleaded guilty to planning the attack with with Medunjanin and testified against him.

Medunjanin sought to join the Taliban, but ended up being recruited by al Qaeda to perform a suicide mission in the United States. On their return, Medunjanin and his two friends hatched a plan to rig backpacks with explosives and blow them up, prosecutors said.

At trial, Assistant U.S. Attorney Berit Berger said Medunjanin was willing "to strap a suicide bomb to himself, walk into a New York City subway and blow it up."