Iran's oil and gas reserves rose by half a billion barrels and 5 trillion cu ft respectively with the discovery of new onshore oil and gas fields in the south and west of the country, an Iranian daily quoted a senior oil official as saying.

The report did not specify whether the oil and gas discoveries were in-place or recoverable.

Iran, the world's fifth largest oil producer, raised the estimate of its oil reserves from 138 billion to 150 billion barrels last year after Iraq upped its reserves estimate to 143 billion barrels, surpassing Iran.

Iran's estimates of gas reserves, almost half of which lie in the giant South Pars gas field in the Gulf, has been put at 33 trillion cubic meters.

Head of the exploration office at the state National Iranian Oil Company (NIOC), Mahmoud Mohaddes, said the new discoveries pertained to five fields in the southern Fars province and the western regions bordering Iraq.

'Details on the discovery of oil and gas in these fields will be announced officially by the end of the year,' the Kayhan daily quoted Mohaddes as saying, adding: 'altogether, the volume of crude oil in these fields has been estimated at more than half a billion barrels.'

Mohaddes said the volume of gas reserves in these fields had been initially estimated at five trillion cubic feet, but it was expected to be increased with more exploratory works under way.

He said the bulk of Iran's exploration efforts were focused on Iran's border regions and those jointly owned with the neighbouring countries as part of the Oil Ministry's policy of prioritizing these regions.

Iran is hoping to discover 2.5 billion barrels of recoverable oil and 23 trillion cubic feet of gas in the course of the country's fifth five-year development programme (2010-2015) now under way.

Iran boasts the world's second-largest natural gas reserves after Russia, but US and UN sanctions deterring investment by Western firms with expertise and technology have slowed its development as a major exporter.-Reuters