US Secretary of State John Kerry said Monday that any attempt by Syrian leader Bashar al-Assad to be re-elected would extend the country's civil war.

"If he thinks he's going to solve problems by running for re-election, I can say to him, I think that certainly this war will not end as long as that's the case that he's there," Kerry said after talks with Arab League officials in Paris.

His comments came as Assad told Lebanese television he did not see "any reason" why he should not run in the next election.

But Kerry said Syria's opposition would never agree to Assad staying in power.

"I don't know anybody who believes the opposition will ever consent to Bashar al-Assad being part of the government," Kerry said.

"He has bombed and gassed people in his country... How can that man claim to rule under any legitimacy in the future?"

Ahead of talks between Western and Arab powers and the opposition in London on Tuesday, Kerry said Washington would continue to support moderate elements among those opposed to the regime.

"We are focused on assisting the moderate opposition," he said. "We will continue... because we believe you need to get to negotiations."

Western and Arab powers are pushing the opposition to take part in peace talks planned for Geneva next month, though several rebel groups on the ground have rejected negotiations.

Asked about Iran taking part in the Geneva talks, Kerry said Tehran would first need to accept the principle of a transitional government agreed at a first round of talks in the Swiss city last year.

"Iran has not accepted the implementation of Geneva 1, so it's very hard to see how it can be constructive," Kerry said.

"If they accept Geneva 1 and want to be constructive... that's a different issue."