US President George W. Bush won backing for tighter sanctions on Iran over its nuclear programme and secured a pledge to send more troops to Afghanistan.
After talks with Mr Bush on the last day of the president's farewell tour through Europe, Prime Minister Gordon Brown said he expected European states to agree to impose some financial sanctions on Tehran and work to approve more.
"So today, Britain will urge Europe and Europe will agree to take further sanctions against Iran," he told a news conference.
"First of all we will take action today that will freeze the overseas assets of the biggest bank in Iran, the Bank Melli."
A British Foreign Office spokesman later clarified that EU states had agreed to a freeze on Bank Melli's assets and were working on technical steps for it to be put in place, but that there were still discussions for more sanctions.
"We're discussing with the US and EU partners a range of additional sanctions, including the oil and gas sector," the spokesman said. "Those discussions are ongoing."
Iran has ignored calls to stop enriching uranium, a process that can be used to make nuclear weapons, and says its nuclear energy programme is intended only for civilian purposes.
Tehran again ruled out suspending enrichment on Saturday, ignoring political and economic incentives put forward by six world powers including the United States and Britain.
"We will do everything possible to maintain the dialogue, but we are also clear that if Iran continues to ignore united resolutions and continues to ignores our offers of partnership, we have no choice but to intensify sanctions," Mr Brown said.