The United States is preparing to send additional troops to the Middle East in response to mounting concerns over Iran, which Washington blames for attacks on oil tankers last week, two US officials told Reuters on Monday, speaking on condition of anonymity.

The officials did not say how many troops would be deployed or detail the timing of the deployment, which has not been previously reported. If confirmed, it would be in addition to the 1,500 troop increase announced last month in response to tanker attacks in May that it also blamed on Iran.

The Pentagon declined to comment. It was unclear when a new deployment might be announced.

US concerns about the threat to US forces and interests in the region have increased steadily in recent weeks, particularly after the attacks on two oil tankers at the entrance to the Gulf on Thursday.

The United States last week released a video it said showed Iran's Revolutionary Guard were behind Thursday's attacks near the Strait of Hormuz on the Norwegian-owned Front Altair, which was set ablaze, and the Japanese-owned Kokuka Courageous.

The US military released additional imagery on Monday.

"Iran is responsible for the attack based on video evidence and the resources and proficiency needed to quickly remove the unexploded limpet mine," Central Command said in a statement.

Iran strongly denies the accusations.