"The coming days will see these terrorist groups carrying out other attempts whenever they get a chance."
Around 30,000 troops and police have been deployed to protect pilgrims.
Mohammed al-Moussawi, the head of the provincial council, estimated that seven million pilgrims had visited the city in the past 10 days, including from regional countries and Iran.
He said motorbike-propelled carts had been banned from the city. Twenty-five medical teams and 10 mobile clinics have been deployed with 65 ambulances on call.
Two units of counterterrorism forces have been sent to the city from Baghdad.
Despite tough security measures, a female suicide bomber killed more than 40 pilgrims on Monday as they were walking from Baghdad to Karbala.
Another 23 were killed in Karbala on Wednesday when a bomb on a cart tore through a crowd.