By conservative estimates, 1 million photographs of child pornography are on the Internet, with an additional 50,000 being posted every year, said Cecilia Malmstrom, the European Union's commissioner for home affairs who was one of sponsors of the conference in Brussels.
"Behind each of these images there is an abused child, an exploited and helpless victim," Malmstrom said at a news conference. "And every time someone looks at these pictures, that child is exploited and violated again and again and again."
The alliance will focus on identifying and helping victims, prosecuting offenders, increasing public awareness and reducing the availability of child pornography online, according to a joint declaration.
The countries will also work on exchanging information, expertise and training, U.S. Attorney General Eric Holder said.
"It's only by working together - in common cause, in good faith, and with mutual respect - that our nations will be able to secure the progress we need in bringing criminals to justice and getting abused and exploited children to safety," he said.
"Those who would harm our children are not respecters of our national borders - we in law enforcement must not allow these borders to be impediments to our best efforts," he said.