Geneva__ The United States and five world powers are looking to reach agreement on the first phase of a two-part nuclear deal with Iran, that would halt the expansion of Iran's nuclear program while negotiations on a comprehensive final deal take place, a senior US official said here Wednesday.

"The first step understanding would put time on the clock to negotiate a final agreement that addresses all of our concerns," the senior US administration official, speaking not for attribution, told journalists here Wednesday on the eve of two days of nuclear talks. "It's crucial we have this phase of this agreement without Iran's nuclear program marching forward."

The US official argued forcefully against Congress implementing new sanctions at this time, saying US experts believe new sanctions "would be harmful to...and undermine" the negotiating process. "We all have an obligation not to take that risk."

"For the first time, Iran appears to be committed to moving this negotiating process forward quickly," the official added. "That's a key shift... For the first time, we do not see them using the negotiating process to buy time."