In this photo released by the official website of the office of the Iranian Presidency, President Hassan Rouhani sits at the Mehrabad airport pavilion before leaving Tehran, Iran, for New York to attend United Nations General Assembly, Monday, Sept. 23, 2019. (Iranian Presidency Office via AP)

NEW YORK (AP) — Britain has concluded that Iran was responsible for attacks on oil facilities in Saudi Arabia, U.K. Prime Minister Boris Johnson said Sunday. He said the U.K. would consider taking part in a U.S.-led military effort to bolster the Gulf kingdom's defenses, while Iran's president announced plans for a rival Iranian-led security coalition.

The U.K. Conservative prime minister also said the he would work with allies to "de-escalate" Middle East tensions that have soared since the Sept. 14 attack on the world's largest oil processor and an oil field.

Britain had previously held back from attributing blame for the drone and missile attack. Saudi Arabia and the United States say Iran was responsible, something Tehran denies.

Johnson told reporters flying with him late Sunday to New York for the U.N. General Assembly that now "the U.K. is attributing responsibility with a very high degree of probability to Iran" for the attack by drones and cruise missiles.

"We will be working with our American friends and our European friends to construct a response that tries to de-escalate tensions in the Gulf region," Johnson said.

Iranian Foreign Ministry spokesman Abbas Mousavi later criticized Johnson's comments and said Britain should stop selling arms to Saudi Arabia amid its war in Yemen, the semi-official ISNA news agency reported.