The US government is trying to bring back nuclear safety employees it fired on Thursday, but is struggling to let them know they should return to work, NBC News has reported.

The National Nuclear Security Administration workers were among hundreds of employees in the energy department who received termination letters.

An email obtained by NBC said the letters for some NNSA employees "are being rescinded, but we do not have a good way to get in touch with those personnel".

The terminations are part of massive effort by President Donald Trump to slash the ranks of the federal workforce, a project he began on his first day in office, less than a month ago.