Congressman Ed Royce, Chairman of the powerful House Foreign Relations Committee's comments came after the deadly shooting in California on December 2 by a radicalised Pakistani-origin couple that killed 14 people.
He alleged that the madrassas were spewing hate.
"Unless that infrastructure of hate is shut down, Pakistan will never win its struggle for internal peace," Royce said during a Congressional hearing on Pakistan.
"That's the issue. We have the list of the 600 schools. I've made three trips, as I've indicated, to try to convince the government to shut those down. We've had little success in convincing families in the Gulf states not to send their money there or convincing those governments in the Gulf states not to fund this," Royce said.
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Responding to his concerns, Richard Olson, Special US Representative for Afghanistan and Pakistan, said the growth of such madrassas is due to the lack of primary education system.
"The reason, in a way, that they exist and have become popular in Pakistan, if that's the word, is because they do provide a free education," he said.
"So there's a real draw factor in all of this. We also think that it's important that the government of Pakistan, and we're working in this area, in the countering violent extremism area, to try and reform the curriculum so that at least in the religiously-oriented schools, there are marketable skills, standardised curricula and there are attempts to address a more modern perspective," Olson said.
"And if they do it funding public education there for individuals, for families, as an alternative for their sons to go to those schools in this case, instead of the lads going to schools where you and I suspect the final outcome is going to be like a lot of others that were radicalised in those Deobandi schools," he said.