(CNN) Deeming congressional rejection of his border national emergency "reckless" and "dangerous", President Donald Trump issued the first veto of his presidency Friday, insisting the situation on the southern frontier amounted to a threat to Americans' safety.

"Congress has the freedom to pass this resolution and I have the duty to veto it," Trump said from the Oval Office before officially sending the measure back to Congress without his approval.

Trump said the resolution, which would have reversed the national emergency, "put countless Americans in danger."

"There haven't been too many that are bigger emergency than what we have" at our border, he said.

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