MEXICO CITY--Mexico on Tuesday revised downwards its confirmed death toll from a new strain of flu to seven from 20, Health Minister Jose Angel Cordova told reporters.

Cordova also raised the toll of "probable" deaths from the H1N1 virus to 159 from a previous figure of 152.

A total of 1,311 people remain in hospital after exhibiting symptoms of the flu, he said.

Cordova explained the change in the confirmed death toll by saying further, more rigorous tests had forced an official recount.

"Of 159 (probable) deaths, only seven have been confirmed," he told the journalists.

Under sharp questioning as to how such a discrepancy could have occurred, Cordova said -- to some confusion -- that 26 proven cases of swine flu had been detected, of which "seven corresponded beyond any doubt to deaths caused by the new virus."

He added: "This tallying task has in great part been carried out in conjunction with the WHO (World Health Organization). They only count as 'positives' confirmed cases."

Cordova said Mexican authorities were working with specialized laboratories in the United States and Canada to test suspected flu samples.