A "prolonged and unprecedented" cyber attack has targeted, according to information shared by media outlets on Thursday, Portugal’s General Staff of the Armed Forces (EMGFA) leading to a leak of a variety of secret NATO documents.

Diario de Noticias, a Portuguese newspaper, revealed that the Portuguese government was oblivious to the attack had the US not informed it of the "extremely serious" breach.

According to the outlet's sources, the US intelligence community discovered "hundreds" of classified NATO documents for sale on the dark web. The report the notification of the finding was made straight to the nation's PM Antonio Costa earlier in August.

The US embassy in Lisbon neither corroborated nor refuted the claim, stating that they do not comment on intelligence matters.

The report revealed that the guidelines governing the secure transfer of classified material had been broken and it further read that “This cyberattack was prolonged in time and undetectable and used bots programmed to detect types of document,” noting that documents were extracted over a period of time and in phases.

NATO will demand explanations and guarantees from the Portuguese government, the newspaper revealed, and an unnamed official told responded that the government can guarantee that its armed forces will "work daily so that Portugal's credibility as a founding member of the Atlantic Alliance remains intact."

The data dump comes after NATO said in late August that it was looking into a cyberattack on missile manufacturer MBDA by unknown hostile actors. According to media reports, the hackers sold blueprints for Ukrainian-used weapons on the dark web.

Read more: Hackers sell classified military documents: BBC