* Talks to be in English for first time ever -U.S. official
* As sanctions bite, Iran says it's keen on swift deal
* Israel urges powers not to relax sanctions prematurely
* Talks seen as biggest chance in years to resolve stand-off
* Atmosphere changed since moderate Iran president elected (Updates with start of talks, Salehi, Israeli comments)
By Yeganeh Torbati and Fredrik Dahl
GENEVA, Oct 15 (Reuters) - Iran resumed negotiations with six world powers on Tuesday after a six-month hiatus, under pressure to propose scaling back its disputed nuclear programme to win relief from crippling sanctions.
The two-day meeting, the first since relative moderate Hassan Rouhani was elected Iran's president on a platform to ease its international isolation, is seen as the best chance in years to defuse a long stand-off over Iran's nuclear ambitions that has heightened the risk of a new Middle East war.
In a possible sign of the Islamic Republic's determination to engage meaningfully, the talks in Geneva were expected to be held in English for the first time, said a senior U.S. State Department official, who asked not to be named.
On the eve of the talks, Washington held out the prospect of quick sanctions relief if Tehran moves swiftly to allay concerns about its nuclear programme, although both countries said any deal would be complex and take time.
Western diplomats said it remained unclear whether proposals for ending the dispute that Iran promised to put forward in the meeting would be sufficient to enable headway to be made.
Western powers suspect Iran is trying to develop the means to make nuclear weapons behind the screen of a declared civilian atomic energy programme. Tehran denies this but its refusal to curb sensitive nuclear activity or permit unfettered U.N. inspections has drawn tough international sanctions.
"We definitely hope that the new momentum will translate into some concrete step forward," a senior Western diplomat said ahead of the talks. But Iranian atomic energy chief Ali Akbar Salehi suggested this week's talks may not get that specific.
Asked whether Iran's Fordow underground uranium enrichment site, which the big powers want shut, would be discussed, Salehi was quoted as saying by the Fars News Agency: "We do not expect to get into contents in today's meeting because the discussion will be on the generalities and I believe that the principles, timing and initiating the process will be considered."
CONSIDERING RAPID CHANGES
The U.S. administration official said any potential sanctions relief would be "targeted, proportional to what Iran puts on the table".
"No one should expect a breakthrough overnight," the official said on Monday, speaking on condition of anonymity.
In a hint that Western powers are seriously considering relaxing sanctions in response to any Iranian concessions, leading U.S. and EU sanctions experts came to Geneva to offer help in weighing any potential changes. Diplomats said scenarios for potential relief had been drawn up ahead of the talks.
Israel, widely assumed to be the Middle East's only nuclear power, has warned the West not to scale back sanctions before Tehran has addressed concerns about its nuclear aspirations.
Israeli Prime Minister Benjamin Netanyahu's security cabinet on Tuesday urged the powers to demand a full rollback of Iran's atomic programme. But the cabinet, whose deliberations are usually secret, did not repeat past veiled threats to attack Iran as a last resort to stop it developing a nuclear bomb.
Since 2006, Iran has rejected U.N. Security Council demands that it halt uranium enrichment and has continued to expand its nuclear fuel programme, leading to increasingly harsh sanctions.
Hopes of a negotiated settlement of the dispute were raised last month when President Barack Obama and Rouhani spoke by telephone, the highest level U.S.-Iranian contact since Iran's Islamic Revolution in 1979. Washington and Tehran have had no diplomatic relations since 1980.
Rouhani said in New York last month he wanted a deal with the six world powers - the United States, Russia, China, Britain, France and Germany - within three to six months.
Foreign Minister Mohammad Javad Zarif played down expectations that an agreement would be reached this week.
"Tomorrow is the start of a difficult and relatively time-consuming way forward," Zarif, Iran's chief nuclear negotiator, said on his Facebook page on Sunday.
The U.S. official said the Obama administration was encouraged that Rouhani - whose emollient approach contrasts with the strident anti-Western and anti-Israeli rhetoric of his hardline conservative predecessor Mahmoud Ahmadinejad - had a mandate to "pursue a more moderate course".
In the past, the six powers have demanded, among other things, that Iran halt uranium enrichment, particularly to 20 percent fissile purity, move stockpiles of enriched uranium abroad and close down the Fordow plant, which is buried inside a mountain south of Tehran to help withstand any air attack.
Iran has rejected the demand that it give up any refined uranium but signalled flexibility on other items.
(Additional reporting by Justyna Pawlak, Louis Charbonneau and Stephanie Nebehay in Geneva, Marcus George in Dubai and Dan Williams in Jerusalem; Editing by Mark Heinrich)