Delegates from the P5+1 countries -- the five permanent members of the UN Security Council (Britain, China, France, Russia, the U.S.) plus Germany -- convened Thursday to negotiate a comprehensive agreement on Iran's nuclear program.
A senior unidentified U.S. administration official, speaking on background, said that Thursday's session was "the beginning of a very intense three weeks as we approach the July 20th deadline."
Secretary of State John Kerry acknowledged earlier in the week that the outcome of the negotiations is uncertain because of the discrepancy between Iran's public proclamations and statements in closed-door meetings.
The senior official echoed that assessment.
"There does remain a significant discrepancy, however, between Iran's seeming intent with respect to its nuclear program and the actual content of that program to date. Iranian officials have stated repeatedly and unambiguously that they have no intention of building a nuclear weapon, which is not a hard proposition to prove. All we're asking is for Iran to commit to concrete and verifiable steps to show to the world what they've repeatedly said is indeed true.
"... Ultimately, it is Iran's decision about whether they're willing to give the international community the kind of assurances and verification to match what they say about the peaceful nature of their nuclear program."
Notable deaths of 2014 [PHOTOS]
Syrian Al Qaida group executes Lebanese soldier