The United States called on Turkey and other countries to 'sacrifice' their business ties with Iran to help prevent Tehran from possibly obtaining nuclear weapons.

Iran has defied diplomatic pressure led by the United States and European allies to halt uranium enrichment. Major powers are due to meet in Washington on Friday to discuss a third UN Security Council resolution to toughen sanctions on Tehran.

'The issue of Iran becoming possibly a nuclear weapons power is so important that each country has to make sacrifices,' US Undersecretary of State Nicholas Burns told reporters in Ankara.

Washington is concerned by Nato ally Turkey's announcement that it planned to boost energy cooperation with Iran and invest $3.5 billion in its South Pars gas field starting next year.

'We do not think it makes sense to announce long-term oil and gas deals at a time when Iran is going ahead with nuclear weapons research,' Burns said.

'Business as usual is not the right policy because business as usual allows Iran to escape pressure we think is needed to convince Iran to come to the negotiating table,' he said.

'We are not singling out Turkey for criticism or attention. This is a general US and West European message to many other countries,' he said -- also mentioning Japan, South Korea and India -- before talks with senior Turkish officials expected to focus mainly on Iran.

He said US businesses had made sacrifices by not doing business with Iran and other countries should follow suit.

'This whole trend is away from full commercial engagement and towards sanctions, whether that is UN Security Council sanctions or individual sanctions,' Burns said.

European officials say new EU investment in the Islamic republic is already dwindling because of the political risk and lack of finance for major projects, and exports to Iran are falling as governments and banks cut back trade credits.

European diplomats say the French are pressing for tougher sanctions on credit, banking and insurance with Iran, if possible at the UN Security Council but if not, then through the European Union. For instance they want severely to restrict export credit guarantees.

The 27 EU countries exported goods worth 12.99 billion euros ($18.06 billion) to Tehran in 2005, 11.27 billion euros last year and 4.66 billion euros in the first half of this year, according to the EU statistics office Eurostat.

The biggest exporters were Germany, Italy and France. But in each case, the volume of trade is declining, the figures show.

'Our message to Germany and Italy is also you have to sacrifice here for the greater objective -- to stop Iran from becoming a nuclear weapons power,' Burns said.

 Western powers accuse Iran of using a purported nuclear power programme as a screen for development of nuclear arms -- something they fear could add enormously to instability in the already volatile Middle East. Iran denies the charges. - Reuters