The United States is awaiting a formal UN deadline before seeking any new measures to coerce or punish Iran over its disputed nuclear programme, the State Department says.
"We're looking at the August 31 date and I think once we get there, we'll begin to take action," State Department spokesman Gonzalo Gallegos said on Thursday.
The UN Security Council is set to take up the Iran case on Aug 31, and could soon consider a new resolution to impose economic or other sanctions on Tehran. The US has long favoured sanctions.
The UN Security Council gave Iran until the end of August to stop enriching uranium, a key component of both nuclear weapons and the peaceful nuclear energy that Tehran says it wants.
The suspension is a precondition before Iran could bargain with European nations and the United States over a package offering Iran economic and other incentives if it gives up portions of its nuclear program that the West fears could produce a bomb.
Iran made a counteroffer to the proposal Tuesday that ignores the UN demand on uranium.
Germany said on Thursday that Iran's response appears unsatisfactory, and Gallegos repeated the US assessment that the response "falls short" of UN conditions.
"We've acknowledged that Iran considers its response a serious offer and we're reviewing it," Gallegos said.