Spain was one of the first European nations to suffer a second spike in Covid-19 infections but restrictions such as nighttime curfews helped to halve the incidence level in November.
But the number of daily infections had risen to more than 10,000 on Tuesday from roughly 9,000 at the beginning of December, according to health ministry figures.
“In recent days we have observed a worrying rise in the number of cases,” Sanchez said during a debate in parliament.
The prime minister said he would not hesitate to instruct regional governments, which are responsible for health care, to toughen their Christmas plans.
The rules that have so far been introduced for the Christmas holidays include a cap on the size of indoor gatherings and restrictions on travel within Spain.
“It is up to us to not to open the door to a third wave,” Sanchez said. “We can't let our guard down.”
Spain has been one of Europe's worst-hit countries, with the virus infecting more than 1.7 million people and causing over 48,000 deaths.