Iran is likely to delay the launch of its new oil-and-gas contracts by three months until December, a top oil adviser here said, as the country seeks the best timing to take advantage of a potential lifting of western sanctions. The new contracts are expected to offer more-favorable terms than in the past for international oil companies and were expected to be unveiled officially at a presentation in London in September. The new terms are a closely watched part of Iran's push to lure back foreign firms forced to leave in 2010 when much of the west imposed crippling sanctions on Iran. Now, with a deal that would lift sanctions in exchange for curbs on Iran's nuclear program likely coming after a June 30 deadline, the timing is too short to market the new deals in September, said Mehdi Hosseini, who heads the oil ministry-appointed Oil Contracts Revision Committee, in an interview here in the Iranian capital. "For us, [the date of the presentation] does not make a difference, the issue is so important," Mr. Hosseini said. European sanctions imposed in 2010 have helped slash Iran's production to less than three million barrels a day, about half its peak level in the 1970s. Any agreement with the west is expected to unleash a flood of foreign investment into Iran's oil and gas fields, many of which are old and need advanced production techniques that major international oil companies possess. Any production happening with the help of foreign companies would take years to develop, so a delay of three months to present the contracts likely won't delay the return of Iranian exports to the market. Iranian officials have said they have the ability to increase oil production by a million barrels a day this year if sanctions were lifted New contracts are seen as essential for oil companies looking to return to Iran. While U.S. companies haven't worked in Iran since the 1979 revolution that ushered in the Islamic Republic, European companies such as Total SA (FP.FR, TOT) of France and Eni SpA (E, ENI.MI) of Italy didn't pull out until as late as 2010. Some have said they wouldn't return if Tehran didn't replace so-called buy-back contracts instituted in the 1990s after most failed to yield a profit from them. Those contracts left oil companies on the hook to pay for cost overruns and limited how much they could profit per barrel, no matter the price of oil. Iranian officials have said the new contracts would pay companies more for boosting production, last longer and encourage joint ventures from the start of the project. The companies would also be expected to share more technology and management expertise with Iranian partners. With the new contracts, costs can be adjusted year to year though the state-run National Iranian Oil Co. would have a final say on any change, Mr. Hosseini said. "If a company is paying something, they should not be out of pocket," he added. Iran also plans fees per barrel produced but those, too, won't be fixed--depending on the risks involved and fluctuations in oil prices, he said. Mr. Hosseini said the text of the new contracts was "nearly completed." Iranian officials have shared some details of the contracts with international oil companies and touted the country's relatively low production costs--about $10 a barrel compared with $50 or more in the U.S. Mr. Hosseini said two-thirds of Iran's oil and gas resources were still unexplored. One sticking point would be whether companies can count the reserves on their financial statements--a key measure of a company's value to investors. Iranian law prohibits foreign ownership of natural resources. "If the booking of reserves is interpreted as a transfer of title, it's not possible," Mr. Hosseini said.