Iran's foreign minister has returned to the nuclear talks in Vienna where negotiators are struggling to overcome still significant differences and preparing to work through a self-imposed deadline for a deal.

Iranian Foreign Minister Mohammad Javad Zarif was rejoining the talks Tuesday after consultations in Tehran and was to meet U.S. Secretary of State John Kerry. Zarif arrived in Vienna with Ali Akbar Salehi, the head of Iran's atomic agency, who had missed earlier sessions due to illness. Iran's official news agency said Salehi's participation indicated Iran's serious desire to accelerate the talks and achieve a comprehensive deal. Russian Foreign Minister Sergey Lavrov was also expected to join the discussions.

On Monday, U.S. officials suggested that significant backtracking by Tehran's negotiators may need several more days of discussions to resolve.