LONDON (Reuters) - Foreign Secretary Boris Johnson said on Thursday Britain was "open-minded" about the timescale for Syrian President Bashar al-Assad relinquishing power and did not rule out joining Russia in military action against Islamic State militants.

Speaking to British lawmakers, Johnson also questioned whether the new administration of U.S. President Donald Trump fully understood Iranian involvement in Syria and the value of a nuclear deal struck between Tehran and world powers.

"There are no good options here (in Syria). We've been wedded for a long time to the mantra that Assad must go and we haven't been able at any stage to make that happen," Johnson told the House of Lords committee on international relations.

"If there is a possibility of an arrangement with the Russians that simultaneously allows Assad to move towards the exit and diminishes Iranian influence in the region by getting rid of Assad and allows us to join with the Russians in attacking Daesh (IS) and wiping them off the face of the earth ... then that might be a way forward."

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Britain is part of the U.S.-led coalition involved in air attacks on IS in Syria and Iraq, and the government's position has been that no solution to the Syrian conflict is possible without the removal of Assad. British ministers have also been critical of Russia's military intervention in support of Assad.

"REALISTIC"

But Johnson said there was a need to be "realistic about the way the landscape has changed" and to think afresh, saying it was conceivable that Assad could stand in a future election.

"It is our view that Bashar al-Assad should go, it's been our long-standing position. But we are open-minded about how that happens and the timescale on which that happens," he said.

Syria's President Bashar al-Assad speaks to French journalists in Damascus, Syria, in this handout picture provided by SANA on January 9, 2017. SANA/Handout via
Reuters

Johnson said the Trump administration should recognize that any deal with Russia on ending the Syrian conflict would also involve "an accommodation with Iran", another key Assad ally.

Johnson praised the 2015 nuclear deal with Iran, which Trump has called "the worst deal ever negotiated" and has threatened to renegotiate.

"We think that trying to improve relations with Iran through this deal, and it's a pretty cautious thing, is on the whole a good thing and we regard that as one of the achievements of the (former U.S. President Barack) Obama administration."

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Johnson's comments came a day before British Prime Minister Theresa May was due to become the first leader to meet Trump following his inauguration.

(Reporting by Michael Holden; Editing by Gareth Jones)