French authorities are investigating a new coronavirus variant found in the western Brittany region that is more difficult for tests to detect, though for now it does not appear to be more dangerous or contagious.

Stephane Mulliez, director of the regional ARS health service, said at a press conference Tuesday that a hospital biologist in Lannion, a city on the English Channel, sounded the alert after studying dozens of recent deaths.

Seven of the victims had shown classic coronavirus symptoms despite having negative PCR tests, which analyse nasal swabs and are usually highly accurate.

Further testing, of blood as well as mucus samples from deeper in the respiratory tract, confirmed the Covid diagnosis in the patients, who were all elderly and had underlying health risks, Mulliez said.

"One possibility is that the virus spreads more quickly between the upper respiratory tract and the lower portions," Alain Tertre, a regional director of the national Sante Publique France health agency, said at the conference.

Samples sent to the Pasteur Institute in Paris for genetic sequencing confirmed it was a previously unknown variant of the virus that has killed more than 90,000 people in France.

Testing and tracing efforts have been stepped up in response, Mulliez said, to determine if the variant has spread to other regions.

Because it is harder to detect, the ARS has added it to the WHO's list of variants under investigation (VUI), but for now it is not considered worrying enough to be placed in the variant of concern (VOC) category.

"Initial analysis… does not suggest increased danger or contagiousness compared with the original virus," the ARS said in a statement.

For now only the British, Brazilian and South African variants are on the VOC list, out of hundreds that have been discovered by researchers worldwide.

But Prime Minister Jean Castex told lawmakers Tuesday that the widespread prevalence of the three variants meant France was now in "a sort of third wave" of the pandemic, following the surge in cases last March and then again in the autumn.

"The outbreak is going into extra time," he said in Parliament, with Paris hospitals in particular at capacity for Covid patients in intensive care.

President Emmanuel Macron is meeting Tuesday with experts on his Covid advisory panel, ahead of a cabinet meeting on the health crisis Wednesday, as speculation grows that new restrictions may be imposed for the Paris region.