The statement came as world powers were engaged in "serious" talks with Iran over its nuclear programme, media reported. (Agencies)
Kerry made the remarks after three-day intensive talks in Geneva between Iran and Britain, China, France, Russia and the US plus Germany, the so-called P5+1, narrowed their differences but failed to produce an interim deal amid hyped expectations.
The two sides agreed to meet again on November 20 at the conclusion of their talks on Saturday.
"This is the first time that the P5(1) had come together with this kind of a serious set of possible options in front of it, with a new Iranian government," Kerry said in Geneva.
"This is a new overture, and it has to be put to the test very, very carefully," he said.
"Some of the most serious and capable, expert people in our government, who have spent a lifetime dealing both with Iran as well as with nuclear weapons and nuclear armament and proliferation, are engaged in our negotiation," he added. "We are not blind, and I don't think we're stupid."
Israel and some hawkish US lawmakers have inveighed against the proposed deal with Iran, in particular the potential relaxation of some sanctions against Tehran.
On the proposed accord with Iran, Kerry said "It is a first step in an effort that will lock the programme in where it is today in fact, set it back while one negotiates the full deal."
"And there will not be a relaxation of the pressure," he added. "Nobody has talked about getting rid of the current architecture of sanctions. The pressure will remain."
The statement came as world powers were engaged in "serious" talks with Iran over its nuclear programme, media reported.