Iran nuclear talks are in danger of busting through their second deadline in a week, raising questions about whether world powers can cut off all Iranian pathways to a bomb through diplomacy — and testing the resolve of U.S. negotiators to walk away from negotiations.

With a Tuesday target date set for a deal setting a decade of restrictions on Iran's nuclear program, U.S. Secretary of State John Kerry, Iranian Foreign Minister Mohammad Javad Zarif and other top diplomats were huddling in Vienna looking for a breakthrough.

All have spoken of differences remaining, despite 11 days of discussions. And for Kerry and his team, pressure is increasing from skeptical U.S. allies and Congress.

Diplomats had sought agreement by June 30, after missing previous deadlines in July 2014 and last November.