Urgent diplomatic talks are being held in a bid to reach a nuclear deal with Iran, with a deadline to secure an agreement fast approaching.
Significant differences reportedly remain between the two sides meeting in the Swiss town of Lausanne.
For nearly a week the United States, Britain, France, Germany, Russia and China have been trying to break the deadlock in negotiations aimed at stopping Iran from having the capacity to develop a nuclear bomb, in exchange for an easing of international sanctions that are crippling its economy.
But stumbling blocks of enrichment research and the speed of lifting sanctions is threatening to scupper an agreement that could end a 12-year standoff between Iran and the West over Tehran's nuclear ambitions and reduce the risk of another Middle East war.
Meetings are being held in an attempt to try and bridge the remaining gaps by the deadline of midnight local time on Tuesday (11pm UK time).
The aim is to reach an understanding that could serve as the basis for a final accord to be reached by the end of June.
All sides say an agreement is possible but uncertain.
It is unclear what missing the 11pm deadline would mean for the talks which have lasted nearly two years and been extended twice since an interim agreement was reached in November 2013.
Most of the parties have indicated they do not want another extension, although they have also said the interim agreement will remain in place until 1 July, suggesting talks could continue.
As talks resumed on Tuesday, the spokeswoman for the US State Department tweeted: "Long day ahead."
On Monday night, Secretary of State John Kerry admitted: "There are still some tricky issues.
"We are working very hard to work those through. We are working into the night."
The six powers want more than a 10-year suspension of Iran's most sensitive nuclear work.
Tehran, which denies it is trying to develop a nuclear weapons capability, is demanding a swift end to sanctions in exchange for temporary limits on its atomic activities.
Iran says it wants nuclear enrichment only for energy, science, industry and medicine.
But many countries fear Iran could use the technology to make weapons-grade uranium.
There is a sense of added urgency with the US Congress warning it will consider imposing new sanctions on Iran if no agreement is reached this week.
In turn, US President Barack Obama has threatened to veto any sanctions moves by the Republican-dominated Congress.
Israeli Prime Minister Benjamin Netanyahu, who opposes the negotiations, said the agreement being put together in Lausanne sends the message "that Iran stands to gain by its aggression".
Iran said the key issue was lifting sanctions quickly.
The country's nuclear negiotiator Majid Takhteravanchi said: "There will be no agreement if the sanctions issue cannot be resolved.
"This issue is very important for us."
