Health Minister Marta Temido announced Thursday that the government has decided to order a so-called civil requisition, whereby workers in essential services are required by law to report for duty in "exceptionally serious" circumstances.
Hospitals say the strike has forced the cancellation of hundreds of scheduled operations.
Temido says the government had no choice and did not take the decision lightly.
The nurses are pressing their demands for better pay, fewer working hours and lower retirement ages.
Nurses' representatives did not immediately say whether they would challenge the government's order in the courts.