Special Operations commanders have also been updating plans for cross-border raids, which would require approval from President Obama. For now, officials said, it remains unlikely that the United States would make good on such threats to send American troops over the border, given the potential blowback inside Pakistan, an ally.
But that could change, they said, if Pakistan-based militants were successful in carrying out a terrorist attack on American soil. American and European intelligence officials in recent days have spoken publicly about growing evidence that militants may be planning a large-scale attack in Europe, and have bolstered security at a number of European airports and railway stations.
"We are all seeing increased activity by a more diverse set of groups and a more diverse set of threats," said Homeland Security Secretary Janet Napolitano before a Senate panel last week.