A 3-month-old girl and her 5-year-old sister were rescued in Denver by undercover agents during this month's FBI-led sting operation against sex traffickers, CNN reported.

In announcing details of Operation Cross Country XI, FBI officials said 82 other juveniles were rescued during a nationwide sweep last week. The FBI said 120 people were arrested.

In the case of the two sisters in Denver, the alleged trafficker was a friend of the children’s family who made a deal to sell them for sex for $600, the FBI said. The friend communicated with an undercover agent.

"Unfortunately, the number of traffickers arrested -- and the number of children recovered -- reinforces why we need to continue to do this important work," FBI Director Christopher Wray said in a statement.

The average age of the victims recovered from this year's operation is 15, the FBI said. Traffickers would offer the victims to outsiders from hotels, truck stops or from online, the FBI said.

On Oct. 13, the second day of the operation, a minor was rescued by the FBI in El Paso, Texas, when a 16-year-old female victim was advertised online for "entertainment," CNN reported.

The girl was accompanied by a 21-year-old female who offered an undercover agent sexual intercourse with both her and the underage victim for $200, the FBI said.

At a news conference in Denver on Thursday, District Attorney George Brauchler reminded traffickers that undercover agents and task force officers are always lurking, whether behind cameras, on the Internet or in a casino, prepared to make another bust.

"To those out there who are watching this who might avail themselves at the opportunity to exploit children or to engage in even the adult sex trade, this is a bit of a warning. ... The people that you can't see behind those cameras, they are fins. They are fins in the water and they represent sharks who are there all the time,” Brauchler said.

"And my advice to you is to stay out of those waters where the kids are, to stay out of those waters where exploited adults are for sex, because they are out there and they will get you. And when they get you, my office will handle the rest."