Russian regulators have approved the proposed $2.6 billion acquisition of Russia-focused British oil exploration company, Imperial Energy Plc, by Oil & Natural Gas Corporation (ONGC).

Reports quoting sources at the Federal Anti-Monopoly Service, Russia's anti-trust office, said the acquisition ``has already been approved.''

However, a meeting between Russian prime minister Vladimir Putin and petroleum minister Murli Deora on Wednesday failed to reach a firm backing for ONGC's acquisition of Imperial Energy, sources said.

Russian energy minister Sergy Shemato would be visiting New Delhi at the end of this month to hold further talks.

''We have done our job. We cleared the deal after the ministry of natural resources deemed Imperial Energy's assets non-strategic,'' agency spokesman Sergei Noskovich said.

ONGC, India's biggest oil producer, had, in August, proposed to acquire the London-based Imperial Energy Plc for 1250 pence a share, valuing it at around $2.6 billion.

Once the deal goes through, the deal would mark ONGC's second investment in Russia, where the company already has a 20 per cent stake in the Sakhalin-1 oil and gas consortium headed by US major Exxon.

ONGC beat China's Sinopec to clinch the deal. Imperial Energy has assets mainly in the Tomsk region of Siberia.

Kremlin, however, is expected to ask ONGC to sell a stake to Russian state oil group, such as Rosneft.

Russian energy majors had taken stakes, sometimes controlling stakes, in almost all foreign energy ventures in the country, making way for state-backed giants winning major stakes in big, formerly privatised energy assets.

While the move would strengthen Russian ties with major Asian energy consumers, the location of Imperial's fields would make it difficult for ONGC to ship oil to India.

Imperial shares rose 23 per cent, trading at 1,090 pence in early morning trade,  while ONGC shares were up 2 per cent at Rs758, still short of ONGC Videsh's offer price of 1,250 pence a share.

Deora also raised the issue of ONGC Videsh Limited (OVL) participation in the Sakhalin oil field projects. OVL has 20 per cent stake in Sakhalin-I fields.

While Sakhalin-I and II projects have already been decided, Russia is planning to invite bids for Sakhalin-III and IV projects in the vast energy-rich region.

India is also reported to be keen on OVL forming a 49:51 joint venture with either Rosneft or Russian gas giant Gazprom to pursue other oil and gas exploration opportunities there.