Iraq has "credible" intelligence that Islamic State militants plan to attack subway systems in Paris and the United States, Prime Minister Haider al-Abadi said on Thursday, but American and French officials said they had no evidence to back up his claims.

Al-Abadi's comments were met with surprise by security, intelligence and transit officials in both countries. New York's leaders scrambled to ride the subway to reassure the public that the nation's largest city was safe.

Al-Abadi said he received the information Thursday morning from militants captured in Iraq and concluded it was credible after requesting further details. The attacks, he said, were plotted from inside Iraq by "networks" of the Islamic State, which was previously known as ISIL.