US Secretary of State Antony Blinken on Monday renewed his vow of a "collective response" to Iran, which had warned adversaries against reprisals after Tehran was blamed for an attack on an Israeli-linked tanker.

"We are in very close contact, in coordination, with the United Kingdom, Israel, Romania and other countries. And there will be a collective response," Blinken told reporters.

Blinken repeated that the United States had concluded that Iran was behind the drone attack off the coast of Oman on the MT Mercer Street, which is managed by prominent Israeli billionaire Eyal Ofer.  

A British security guard and a Romanian crew member were killed.

Blinken called the incident a "direct threat to freedom of navigation and commerce" but played down suggestions that it showed a hardening under Ebrahim Raisi, an ultraconservative who is set to be sworn in on Tuesday.

"We have seen a series of actions taken by Iran over many months, including against shipping, so I'm not sure that this particular action is anything new or augurs anything one way or another for the new government," Blinken said.

"But what it does say is that Iran continues to act with tremendous irresponsibility."