The group had announced earlier that it would invite Iran to attend a new session aimed at breaking a deadlock in the talks.
Western powers have accused Iran of seeking to build a nuclear weapon, a claim that Iran denies.
Tehran insists it has the right under the Nuclear Non-proliferation Treaty to develop nuclear power for civilian energy purposes.
"There is nothing more important than trying to convince Iran to cease its efforts to obtain a nuclear weapon," Clinton said.
The Bush administration had pursued a policy of isolating Iran and not attending the P5+1 group's meetings with Iranian officials on the nuclear issue.
However, the Bush administration did send a representative to one such meeting in Geneva last July.
Condoleezza Rice, the former US secretary of state, said Iran had not been serious at that meeting and such contact ceased.
Barack Obama, the current US president, has signalled a willingness for US officials to hold talks with Iran and other powers with poor relations with the US.
In March, Obama recorded a video addressed to the Iranian people in which he appealed for better relations between the two nations, which have not had diplomatic ties since 1979.