India, June 23 -- Before the US military struck deep inside Iran's nuclear establishments in the weekend, damaging three of its uranium enrichment facilities, Iran had reportedly communicated to President Donald Trump that it could activate sleeper-cells in the US.

Both the White House and FBI officials are on high alert for these Iranian sleeper cells, a threat the US Customs and Border Protection (CBP) says has "never been higher".

While the CBP has not identified any credible threat yet, it said "thousands of Iranian nationals" are being documented entering the US illegally.

Sleeper cells are spies or terrorists who lead normal, unsuspecting lives in the US. They do not engage with their surroundings so no one takes their notice.

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