Two major international accounting firms have withdrawn from the Iranian market. The decision by PricewaterhouseCoopers and Ernst & Young means all of the "big four" international auditors have severed ties with the Islamic republic. Losing access to the approval of the major auditors could discourage risk-averse investors.

The US government has been stepping up pressure on major firms to isolate Iran while ratcheting up efforts to force the Islamic Republic to make more transparent its suspected bid to develop a nuclear weapon.

Last week, Congress' Government Accountability Office listed 41 firms that have invested heavily in Iran's energy sector. In recent days, senior officials have pressed Russia and Switzerland to intensify Iran's isolation.

The news agency AP reported last week that Iran would allow United Nations nuclear inspectors greater access to its uranium enrichment activities.