VIENNA: US Secretary of State John Kerry arrived late Friday in Vienna seeking to wrap up a nearly two-year quest for an accord curbing Iran's nuclear ambitions, but with both sides warning a deal is far from done.

Kerry was expected to meet Iranian Foreign Minister Mohammad Javad Zarif on Saturday, three days before a Tuesday deadline for a deal that officials warn may be missed by a few days.

France's Laurent Fabius and other foreign ministers were due to arrive over the weekend.

Some major problems exist which are still blocking the work... but in other areas we have made good progress," Iranian Deputy Foreign Minister Abbas Araghchi told state television from the Austrian capital earlier Friday.

"Overall, the work is moving ahead slowly and with difficulty," he added.

His assessment was echoed by a Western diplomat involved in the talks, who said that several key issues, including future UN inspections and the timing of sanctions relief, remain "extremely problematic".

"The most difficult issues need to be resolved in the coming days: transparency, inspections, PMD (possible military dimensions of Iran's nuclear programme), sanctions... On the major issues there is major disagreement," the diplomat said.

The P5+1 - the United States, China, Russia, Britain, France and Germany - want Iran to curtail its nuclear activities in order to make any push to make nuclear weapons all but impossible.

The deal, it is hoped, would end a standoff dating back to 2002 that has threatened to escalate into armed conflict and poisoned the Islamic republic's relations with the international community.

In return for downsizing its activities and allowing closer UN inspections, Iran, which denies wanting nuclear weapons, would see painful UN and Western sanctions that have choked its economy lifted.

In April the P5+1 and Iran massively raised hopes that a resolution to the long-running crisis was in sight when they reached an accord in Lausanne, Switzerland on the outlines of a deal that they aim to finalise by the end of June.

But on Tuesday Iran's supreme leader, Ayatollah Ali Khamenei, set out key "red lines" for the final agreement which appeared to go against parts of what had been agreed in the Swiss city.

Khamenei said that all economic and financial sanctions imposed by the UN Security Council and the US must be lifted on the same day that an agreement is signed.

Western powers have said that no sanctions will be lifted until the UN atomic watchdog has verified that Iran has taken key steps outlined in the deal.

These include Iran slashing by more than two-thirds the number of uranium enrichment centrifuges, which can produce fuel for nuclear power but also form the core of a nuclear bomb, as well as shrinking its uranium stockpile by 98 percent.

Iran also agreed to change the design of a planned reactor at Arak so that it cannot produce weapons-grade plutonium and to no longer use its Fordo facility -- built into a mountain to protect it from attack -- for uranium enrichment.

In addition Khamenei took issue with the IAEA visiting military sites - vital for a UN probe into allegations of past efforts to develop nuclear weapons -- and with the time periods for which Iran will halt certain activities.

'Whack-a-mole'

Even if negotiators manage a deal, it will be closely scrutinised by hardliners both in Iran and the United States, as well as Iran's regional rivals Israel, widely assumed to have nuclear weapons itself, and Saudi Arabia.

On Wednesday, Kerry warned that despite the huge diplomatic effort to get a deal - talks began in earnest back in 2013 -- success was not guaranteed.

If Iran cannot address the remaining issues needed to assure the world its nuclear programme is peaceful, "there won't be a deal," he said.

On Thursday a senior US official said that the key unresolved points in what will be a highly complex accord included the timing and pace of sanctions relief and details about access and transparency for UN inspectors.

"These negotiations have been extremely tough," the official said. "It is like (the game) whack-a-mole. Another issue may pop back up because you've changed the balance of the deal because of any decision."

By AFP