U.S. Secretary of State Condoleezza Rice said on Thursday she did not know if Iran would respond positively this weekend to an offer made by major powers to suspend sensitive nuclear work in exchange for incentives.

"I don't know," Rice told reporters when asked if she anticipated a positive response from Iran during talks in Geneva this Saturday that a senior U.S. envoy is also attending in a break with U.S. policy.

"The point we are making is that the United States is firmly behind this diplomacy and firmly behind and unified with our allies. Hopefully the Iranians will take that message," she added. Rice decided this week to send the third-ranking official in the State Department, William Burns, to join a meeting with Iran's nuclear negotiator along with EU foreign policy chief Javier Solana and other envoys from China, Russia, France, Britain and Germany.

Iran is expected to respond to an offer of incentives made last month by major powers, which includes financial and diplomatic carrots as well as cooperation in developing a civilian nuclear program.