LONDON  Prime Minister Gordon Brown announced Monday that Britain and the European Union would freeze the overseas assets of Iran’s largest commercial bank, joining the United States in intensifying financial pressure against Iran over its refusal to address international concern over its nuclear activities.

Mr. Brown, appearing with President Bush after discussions here, also pledged to send additional troops to Afghanistan, and indicated that he would not bend to political pressure at home to withdraw British forces in southern Iraq more quickly.

Both leaders said that they remained open to resolving the dispute with Iran diplomatically, but only after it suspended uranium enrichment, which Mr. Bush and others have said is aimed at developing nuclear weapons. Iran’s leaders insist the nuclear program is for peaceful purposes.

Iran gave a chilly reception to a proposal from Western powers that was delivered Saturday. The proposal offered economic and diplomatic incentives to Iran to suspend its enrichment and to begin talks to resolve questions about its nuclear work. While Iran has not formally responded, President Bush said Saturday that it had rejected the offer.