Iran has cancelled an oil conference in London next month, when Tehran was expected to reveal its new oil and gas contracts to investors, apparently over a delay in getting visas, the Seda weekly on Saturday quoted a senior Iranian official as saying.

The deputy head of the National Iranian Oil Co. (NIOC) also said foreign companies would be invited in May to bid for new oil contracts.

"The London conference has been cancelled because the British embassy in Tehran could not issue visas for representatives of Iranian companies as its visa section has yet to become active," Ali Kardor told the weekly. "And they had to acquire their visas from a third country."

The conference has been postponed five times because of uncertainty over sanctions. In September, Iranian oil ministry officials said the Iran Petroleum Contract (IPC) would be revealed in London in Feb. 22-24.

 Sanctions lifted

 International sanctions on Iran were lifted earlier this month as part of a nuclear deal reached with six major powers in 2015 in exchange for curbing Tehran's disputed nuclear work.

To bolster its sanctions-hit economy, Iran has sweetened the terms it offers on oil development contracts to lure back international oil companies deterred by sanctions.

The OPEC producer has said it plans to increase its oil output by 500,000 barrels per day (bpd) and by another 500,000 bpd shortly after that to boost exports.

Some 135 energy companies, including British Petroleum , France's Total, Italy's Eni and Spanish oil major Repsol attended a conference in Tehran in November to hear about the IPC.

Some details of the new contracts were revealed to major companies, but Iranian officials said more details would emerge in London.

The IPC offers more flexible terms on price fluctuations and investment risks, ending a more than two-decade old buy-back system that banned foreign companies from booking reserves or taking equity stakes in Iranian companies.

Under the new model, the NIOC would have ownership rights over resources and foreign companies would still not own oilfields. In some cases, it allows reserves to be booked by foreign companies.

When asked whether the cancellation of the London conference was "partly because of mounting pressure by hardliners" in Iran, the official said :"We are in talks with critics to hear their points and to resolve problems when needed."

Pragmatist President Hassan Rouhani's hardline rivals have harshly criticised the new contracts.

Some analysts say Rouhani's government is determined to resolve the dispute through diplomacy.

"The oil ministry and the government want a political consensus over this issue ... in a public statement, the government has asked all the experts to express their views," said Tehran-based senior energy analyst Reza Zandi.

A draft of the IPC was approved by the government in September, but the official said the new contract should be approved by the NIOC's board of governors.

"There will be bids in May for new contracts ... There will be no need to hold a conference abroad after the bids," he said.

"The bids will be announced through the usual legal procedure ... joint oil fields with Iran's neighbouring countries have priority for us."