* UK believes Iran was behind Saudi attack
* Military action? Johnson will consider proposals
* Houthi's claim 'implausible' - UK official
* Iran authorised attack - UK official (Recasts headline, adds UK official)
By Kylie MacLellan
NEW YORK, Sept 23 (Reuters) - Britain believes it is very likely Iran was behind an attack on Saudi oil facilities and London will work with the United States and European allies to de-escalate tensions in the Gulf, Prime Minister Boris Johnson said on Monday.
The United States and Saudi Arabia have already blamed Iran for the Sept. 14 strikes on two Saudi Aramco oil plants that initially halved the country's crude output. Yemen's Iran-aligned Houthi movement has claimed responsibility.
"The UK is attributing responsibility with a very high degree of probability to Iran for the Aramco attacks. We think it very likely indeed that Iran was indeed responsible," Johnson told reporters on a plane to the United Nations General Assembly in New York.
"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," he said.
Asked whether Britain would rule out military action, Johnson said it would be closely watching a proposal by the United States to do more to help defend Saudi Arabia.
"Clearly if we are asked, either by the Saudis or by the Americans, to have a role then we will consider in what way we could be useful," he said.
The United States is aiming to avoid war with Iran and additional troops ordered to be deployed in the Gulf region are for "deterrence and defense", U.S. Secretary of State Mike Pompeo said on Sunday.
Johnson said he would be discussing Iran's actions in the region with President Hassan Rouhani at the U.N. meeting, as well as pushing for the release of several dual national Iranians who he said were being held "illegally and unfairly".
A British government official said the Houthi's claim of responsibility was "implausible" because the scale, sophistication and range of the attack on the Saudi oil facilities was inconsistent with their capabilities.
"The sophistication points very, very firmly to Iranian involvement and I think it is implausible that that wouldn't have been authorised by the Iranian government," the official said.
British Foreign Secretary Dominic Raab also said it was very likely Iran was responsible for the attack.
"UK believes it is very likely that Iran was responsible for the outrageous, unlawful attack on oil facilities in Saudi Arabia," Raab said on Twitter.
"We will work with our international partners on a robust diplomatic response and for stability in the region." (Reporting by Kylie MacLellan; editing by Guy Faulconbridge and David Clarke)