Iran has delayed until later in the year a widely anticipated London conference during which it would unveil new oil and gas investment opportunities, two industry sources said.

The conference, which was due to be held on February 23-25, has been delayed twice before, a move apparently aimed at giving time for sanctions on the country's oil sector to be lifted.

Tehran wants Western oil companies to revive its giant, ageing oilfields and to develop new oil and gas projects and has been preparing a new investment model for oil contracts as part of its drive to win back Western business.

The Opec' producer is in talks with six Western powers to reach a deal to limit Tehran's nuclear programme - which could end sanctions on oil investment and trade with Iran.

The deadline to reach a deal was extended by seven months to June after the parties failed to reach a comprehensive agreement by the self-imposed deadline of November 24 last year.

One source said the conference was delayed until November. "They have not given a specific reason. What they said is they are still working on the contract," the source said.

Another source familiar with the matter said the summit was delayed until later in the year, adding that it might be premature to hold the conference now given the ongoing nuclear negotiations and uncertainties in the global oil market.

Iranian officials were not immediately available to comment.

Western sanctions imposed in 2012 on Iran for its nuclear programme have choked Tehran's oil production - output is down a million barrels per day (bpd) since the start of 2012 to 2.7 million bpd - and cost it billions in revenues.

Top Iranian officials say the country can raise production to 4 million bpd within six months of sanctions being lifted. Western experts are more conservative, saying 3 million to 3.5 million bpd is more likely.

Encouraged by a preliminary nuclear deal struck between Iran and Western powers, Tehran and Big Oil have wasted no time making contact, in the hope of a full lifting of sanctions.

Iranian Oil Minister Bijan Zanganeh has met with Western oil executives at Opec's previous meetings in Vienna, including Italy's Eni, Royal Dutch Shell and Austrian oil and gas group OMV.

Eni, Shell, and Total have already developed some of Iran's oilfields from scratch.-Reuters