Iran nuclear talks entered a critical week Monday with US Secretary of State John Kerry and his Iranian counterpart sitting down in Switzerland seeking an elusive breakthrough after 18 months of intense negotiations.

Time is running out, however, with Kerry and Mohammad Javad Zarif aiming to agree the outlines of an agreement by the end of the month. A full accord is then due by July 1.

Both men, who began meeting soon after 0700 GMT in a luxury hotel in the lakeside city of Lausanne, are also under intense pressure from domestic hardliners worried they will give too much away.

Speaking in Egypt before travelling to Switzerland, Kerry sought to ease such concerns, saying that the aim is "not just to get any deal, it is to get the right deal".

"If (Iran's nuclear programme is) peaceful, let's get it done. And my hope is that in the next days, that will be possible," Kerry told CBS television.

There were, however, "important gaps," he said.

"Several questions need to be discussed, those where we haven't found a solution yet and also those where we have found solutions but where we need to discuss certain details," Zarif said on Sunday.

Zarif was later Monday due in Brussels to meet his British, French, German and EU counterparts before returning to Lausanne.

Negotiators from the other five powers involved in the talks -- Russia, China, Britain, France and Germany -- were to be involved from Tuesday, according to Iranian officials.

- Rouhani thaw -

The US and Iran have not had diplomatic relations for 35 years but the 2013 election of President Hassan Rouhani resulted in a minor thaw and a diplomatic push to resolve the more than decade-old nuclear standoff.

Under a landmark November 2013 interim deal with the "P5+1" powers, Tehran stopped expanding its activities in return for minor sanctions relief.

Since then the parties have been pushing for a lasting accord.

But to the alarm of Iran's foe Israel, US Republicans and Washington's Gulf allies, the US looks to have abandoned insisting that Iran dismantle all nuclear activities.

Instead it appears prepared to tolerate a small programme under tight controls and potentially shipping abroad some of Iran's nuclear material, possibly to Russia.

In theory this still leaves Iran with the possibility to get the bomb, critics say.

Iran says its nuclear programme is exclusively peaceful.

Last week 47 Republicans took the unprecedented step of writing an open letter to Iran's leaders.

They warned that any nuclear deal could be modified by Congress or revoked "with the stroke of a pen" by whomever succeeds President Barack Obama, a Democrat.

The Obama administration has been trying to dissuade lawmakers from passing legislation that would force the president to submit any Iran deal to Congress for approval.

Obama is sure to veto this but the Republicans are trying to assemble a two-thirds majority in both houses of Congress with rebel Democrats to pass the measure and override the veto.

"Apparently the administration is on the cusp of entering into a very bad deal with one of the worst regimes in the world that would allow them to continue to have their nuclear infrastructure," Senate Majority Leader Mitch McConnell Sunday told CNN.

- 'We need clarity' -

Some progress has been made towards a deal but the two sides remain far apart on several key issues.

These include the future size of Iran's uranium enrichment capacities -- which can make nuclear fuel but also the core of a bomb -- and the pace at which sanctions would be lifted and the accord's duration.

"We need clarity on the way in which sanctions will be lifted and what the guarantees will be for applying the deal," Zarif said.

Two deadlines, in July and November, were missed but in view of the controversy in Washington -- and pressure in Iran on Rouhani to deliver -- extending yet again will be very tough.

"There is no time for additional extensions," Kelsey Davenport, an analyst at the Arms Control Association, told AFP.