* First face-to-face talks since failed Iran meeting
* Syrian National Coalition leaders to meet some ministers
* Britain will press case for more aid for opposition (Add details, quotes)
By Natalie Huet and Arshad Mohammed
LONDON, April 10 (Reuters) - Western and Middle Eastern nations trying to help the Syrian opposition in its war against President Bashar al-Assad will meet in Turkey on April 20, a U.S. official said on Wednesday as G8 foreign ministers gathered in London for a summit.
U.S. Secretary of State John Kerry will attend the meeting of the so-called Friends of Syria "core group" in Istanbul, said the official, who spoke to reporters on condition of anonymity.
The G8 talks, which began in London over dinner on Wednesday and were due to end on Thursday, will also be the first chance for the ministers to discuss face-to-face the failure of last week's meeting in Almaty on curbing Iran's nuclear programme.
North Korean threats of war also will be high on the agenda of the Group of Eight nations - the United States, Britain, France, Germany, Italy, Japan, Canada and Russia - meeting.
Britain was expected to call for more help for the Syrian opposition but there are no signs of a major shift in policy.
Leaders of the Syrian National Coalition (SNC) were present on the sidelines of the G8 meeting and were expected to hold talks with those foreign ministers willing to meet them.
During a lunch meeting earlier in the day, Syrian opposition members said they needed more humanitarian assistance and Kerry talked about the importance of the opposition becoming better organised, a senior U.S. official told reporters.
British Foreign Secretary William Hague, in a statement issued after the talks, said Britain was committed to finding a political solution to the crisis.
"We discussed what further assistance the UK could provide to save lives in Syria, and how we could work together to ensure this support was channelled most effectively," he said.
The United States, which on Feb. 28 said it will for the first time give non-lethal aid to Syrian rebel fighters and more than double its aid to Syria's civilian opposition, has so far chosen not to provide arms to the rebels and did not make any commitments at the lunch, the U.S. official said.
"He didn't promise anything," the official, who spoke on condition of anonymity, told reporters travelling with Kerry.
During a more than one-hour meeting with Russian Foreign Minister Sergei Lavrov, another U.S. official said there was no sign of any change in Moscow's stance on Syria.
"It certainly didn't sound like they have changed their position a lot," said the official, who added that Kerry and Lavrov's talks on Syria were relatively brief and that the Syrian civil war was expected to be among the main topics of conversation at Wednesday's dinner of G8 foreign ministers.
The radicalisation of elements of the divided opposition has tempered the enthusiasm of some Western nations for supporting the rebels militarily. Iraq's al Qaeda wing said it had united with Syria's al-Nusra Front, a kindred group.
Michael Stephens, a Doha-based analyst for security think tank RUSI, said the presence of SNC leaders at the G8 meeting was designed to help the coalition shore up its international legitimacy after the Arab League recognized it as the sole representative for Syria.
"It shows there's a graduated process, where they went from laughing stock to being approved by the Arab League to being listened to by G8 leaders," he said. "There's a groundswell of support that appears to be building up behind them."
France and Britain are expected once again to press the case for amending or lifting an arms embargo on Syria to support the out-gunned rebels and help end a conflict that has claimed an estimated 70,000 lives and displaced millions of people. (Additional reporting by John Irish in Paris; Writing by Maria Golovnina; Editing by Michael Roddy)