Portugal said on Tuesday that it had summoned Iran's ambassador to condemn Tehran's attack on Israel and to demand the release of a Portuguese-flagged ship seized in the Gulf over the weekend.

"The Portuguese government expressed its profound concern over the escalation of conflict in the region, calling for maximum restraint" and "reiterated in a vehement and categorical manner the recent attack against the state of Israel," the foreign ministry said in a statement.

"The meeting also allowed a renewed call to immediately release the MSC Aries ship", a container vessel operated by the Italian-Swiss group MSC that was seized by Iran's Revolutionary Guards on Saturday over alleged links to Israel.

On Monday, Iran said the ship had violated international maritime law and was undergoing "necessary investigations".

"The ship was directed into the territorial waters of Iran because it violated international maritime laws," foreign ministry spokesman Nasser Kanani said.

The ship's crew failed to "adequately respond to questions from Iranian authorities," he told a briefing, adding: "It is certain that this ship belongs to the Zionist regime," referring to Israel.

Both Israel and the United States denounced the seizure as piracy, with Israel also demanding that the Guards be declared a "terrorist organisation" by the European Union.

The ship's seizure took place hours before Iran launched its first-ever direct attack on Israel, launching hundreds of drones and missiles, nearly all of which were intercepted by Israel and its allies.