Brazil's national health regulator said Thursday its decision to reject the Russian-made Sputnik V vaccine was based on the developer's own data, after the latter threatened to sue for defamation.

The Brazilian agency, Anvisa, based the decision on evidence the vaccine carried a live version of a common cold-causing virus -- information it said "was submitted by the Sputnik V vaccine developer itself."

The decision to deny emergency use authorization for the vaccine, announced Monday, has blown up into an all-out international row.

Sputnik V said earlier Thursday it was "undertaking a legal defamation proceeding in Brazil against Anvisa for knowingly spreading false and inaccurate information."

"Anvisa made incorrect and misleading statements without having tested the actual Sputnik V vaccine," it tweeted.

The regulator fired back defensively in a press conference.

"Anvisa was accused of lying, of acting unethically, of disseminating fake news about replicating adenovirus," said the agency's director, Antonio Barra Torres.

"We refute this grave accusation."