Senator Charles Grassley asked Microsoft to provide details on how many non-US citizens it employed under the H-1B visa programme, and what proportion of the staff being let go would come from this group as opposed to people born in the US.
"My point is that, during a layoff, companies should not be retaining H-1B or other work visa programme employees over qualified American workers. Our immigration policy is not intended to harm the American workforce," he wrote.
"I encourage Microsoft to ensure that Americans are given priority in job retention. Microsoft has a moral obligation to protect these workers by putting them first during these difficult economic times."
Grassley is a long-term critic of the H-1B programme, whereby 65,000 visas are issued to foreign workers with needed skills and an additional 20,000 to those born overseas who had completed their degree at a US university.
H-1B has been oversubscribed for some years, and this is expected to continue next year despite technology companies making large numbers of staff redundant.
Former Microsoft chairman Bill Gates has already called for an expansion of the programme, telling the House Committee on Science and Technology that the current limit is "arbitrary" and "bears no relation to the US economy's demand for skilled professionals".
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