WASHINGTON - Hundreds of US, British and Canadian citizens who have trained with Islamic State fighters trying to carve out their own state in Iraq and Syria pose a "very serious threat" to the United States, a top Republican lawmaker said Sunday.

Representative Mike Rogers, chairman of the House Intelligence Committee, urged the Obama administration to aggressively prosecute US citizens who had trained overseas as it weighed options for how to respond to escalating violence by Islamic State militants in Iraq.

He said an attack on the Jewish museum in Brussels in May had shown a new push by extremist groups to carry out attacks on Western targets, and growing efforts to recruit backers on social media.

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