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. 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) will be revealed in London in February 22 to 24.

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.

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.-Reuters