British Foreign Minister William Hague has said global powers negotiating with Iran over its nuclear programme must "seize the moment" as talks enter an unscheduled third day.
Six world powers - the US, Russia, China, Britain, France and Germany - are working on a deal to cap some of Iran's atomic programme in exchange for limited relief from economic sanctions.
As delegates arrived on Saturday, Mr Hague told reporters: "We are very conscious of the fact that real momentum has built up in these negotiations and there is now real concentration on these negotiations and so we have to do everything we can to seize the moment.
However, he cautioned that it was not clear whether a deal could be reached by the end of the day.
France's Laurent Fabius said the sticking points were a call for Iran to halt operations at its Arak research reactor - a potential producer of bomb-grade plutonium - while the negotiating process continues and questions about Iran's stock of uranium enriched to 20%.
Both issues reflect Western concerns that Iran is enriching uranium for use in atomic weapons rather than in a civilian nuclear energy programme as it claims.
US Secretary of State John Kerry, who cut short a Middle East tour to attend the talks in Geneva, Switzerland, had also struck a note of caution after a five-hour meeting drew to a close last night.
Iranian foreign minister Mohammad Javad Zarif (centre) is at the talks "There is not an agreement at this point," Mr Kerry told reporters. "There are still some very important issues on the table that are unresolved."
Iran's deputy foreign minister Abbas Araqchi had stressed: "It was productive but still we have lots of work to do."
Earlier on Friday, Russian foreign minister Sergei Lavrov raised hopes after he said the six countries and Iran could agree a "road map" to end the differences over the programme at the talks.
He told reporters he did not wish to prejudge the outcome but said Iran should be allowed to have a peaceful nuclear programme under the watch of the International Atomic Energy Agency.
Unlike previous encounters between Iran and Western powers in the past decade, all sides have remained quiet about details of the negotiations, without the criticism and mutual allegations of a lack of seriousness that have been typical of such meetings in the past.
Diplomats involved in the talks say this is a sign of how serious all sides are.
If some sort of agreement is reached, it would be a breakthrough after a decade of negotiations between Iran and the six world powers.
A potential deal could see Tehran freeze its nuclear efforts for as long as six months in exchange for some relief from the sanctions that have battered its economy.
But Israel's prime minister Benjamin Netanyahu said on Friday that his country "utterly rejects" a deal being forged, adding that "Israel will do everything it needs to do to defend itself and defend the security of its people".
Some 130 Eritreans were assaulted in Libay according to police 
Those who survived the crossing are held at detention centres in Italy
Police also held this Tunisian man suspected of driving one of the boats
A man walks amid shattered homes in Tacloban
A mother weeps beside the dead body of her son
Children play among downed power lines
Residents carry the body of a loved one
Soldiers walk outside of Tacloban's shattered airport terminal 
Children ran and cowered in terror as the typhoon hit
A map showing the path of the typhoon and affected islands
Taliban leader Hakimullah Mehsud was killed last week
Residents of Legazpi city in Albay province, south of Manila
Damage to buildings in Ormoc City. Credit: Ritchel M. Deleon
Children sheltering in Cebu. Picture: Red Cross
Waves up to six metres high have been reported as a result of high winds
A closer look reveals the eye of the storm over the Philippines
A map showing the path and predicted path of Typhoon Haiyan
Sergei Lavrov is hoping for a "concrete result"
Iran's president denies his country wants to make nuclear weapons
US Secretary of State John kerry
Haiyan is expected to cause massive damage
Russian security services seize Arctic Sunrise
Inside the detention centre in Murmansk
Knox has refused to travel to Italy for the new appeal
Meredith Kercher was found dead in her room in Perugia in 2007
Works have been examined by art historian Meike Hoffmann
Ambulance workers at the scene of the killings
The revelations have led to protests in the US and abroad
The NSA has allegedly listened in on Angela Merkel's phone calls
Morsi supporters have been protesting outside the trial
Riot police are keeping watch at the police academy
Protesters attack a TV truck outside the court
Suspected gunman Paul Ciancia
The attack caused major disruption to travellers
A police officer at Los Angeles airport
Hakimullah Mehsud (c) seen with other Taliban militants in a video in 2009
Survivors liberated from Auschwitz in 1945
How Mr Kempf described the items on eBay
Schindler's list failed to sell on eBay when offered for sale for £3m