Iran's annual oil and gas fair opens in Tehran on Tuesday, with 600 foreign companies seeking to position themselves for a return to large-scale operations if international sanctions are lifted. With some of the world's biggest oil and gas reserves, the industry is the cornerstone of Iran's economy, but it was heavily hit by an American and European embargo on the energy and banking sectors in 2012. Major energy firms have since left or been stifled from doing business because of economic sanctions imposed as punishment for the country's disputed nuclear programme. Asian oil giants have sought to fill the void but Iran cancelled a contract with China National Petroleum Corporation last week, citing the latter's failure to fulfil output requirements. Since an interim agreement with the P5+1 group of Britain, China, France, Russia, the United States and Germany came into effect in January, Iran has been allowed to maintain its oil exports at around 1.2 million barrels per day. - Foreign investment needed - Despite such numbers being higher than in recent years, Iran's oil exports are a fraction of what they were before sanctions -- the country was shipping 2.5 million bpd of crude abroad in late 2011. President Hassan Rouhani and Oil Minister Bijan Zanganeh have said in recent months Iran would benefit from the investment and technology transfer that international oil companies can deliver. Regional representatives from France's Total and the German conglomerate Siemens are among 600 foreign companies registered for the International Oil, Gas and Petrochemical Refining Exhibition, according to government officials. The number is three times higher than in 2013, Nematollahi Akbar, a spokesman for Iran's Oil Ministry, told AFP, noting that 1,200 Iranian firms will attend, up 50 percent. "Total and Siemens will be represented by their regional officials and Chinese companies are present this year," Akbar said. Iran and world powers are preparing for a fifth round of talks in Vienna next week aimed at a long-term deal on curbing Iran's nuclear activities. Nearly one year after the election of Rouhani, a self-declaredmoderate, "the good participation" from foreign companies allows "renewal of relations and the presentation of projects", Akbar said. - Big interest - An Iran-based Western businessman involved in petrochemical engineering told AFP there were high hopes of a return to large-scale operations. "I hope that Western officials will be more accommodating because of the general political climate in Iran," he said, noting his company had maintained a local presence to be ready to react if sanctions are lifted. Hatef Haeri, a Tehran-based energy expert with the ICG consultancy group, said this year's stronger foreign presence at the fair indicated better conditions for deals. "The energy market in Iran is huge," he said. "With a five-year plan for $150 billion of investment, some of this must come from abroad." Iran has the world's fourth highest oil reserves and the second highest gas deposits, behind Russia. However, a second economic expert based in Tehran who will attend the fair, was more cautious, stating the world's biggest oil companies would not send global representatives.