* Vedanta confirms bought $1.5 bln stake at 331 rupees/shr
* $9.6 bln deal to acquire Cairn India awaits govt nod
* Stake purchase eases pressure on open offer for 20 percent
* Cairn India shares rise as much as 3 percent
* Petronas sells entire 14.9 percent stake in Cairn India
(Updates with Vedanta confirmation, details)
By Indulal PM and Sumeet Chatterjee
MUMBAI, April 19 (Reuters) - Vedanta Resources has bought a 10.4 percent stake in Cairn India, pushing ahead with plans to take control of the Indian oil and gas company despite regulatory delays plaguing the $9.6 billion deal.
Vedanta's move to acquire Cairn India is part of London-based mining magnate Anil Agarwal's plans to grab a slice of India's oil reserves and get exposure to surging demand.
But its agreement to buy a 40 percent to 51 percent stake from Cairn Energy has yet to be cleared by the Indian government. London-listed Vedanta said on Tuesday it expected to own between 51 and 70.4 percent once the deal and an ongoing open offer to Cairn India minority shareholders were complete.
Vedanta said on Tuesday that it bought the stake -- 200 million shares -- from Malaysia's national oil corporation Petronas for 331 rupees ($7.40) a share, below the price being offered to minority shareholders in the open offer by Vedanta unit Sesa Goa and reflecting a 1.6 percent discount to Cairn India's closing price on Monday.
Vedanta said it was offered only the shares it eventually bought. Petronas sold the remainder of its stake, around 4 percent, to foreign portfolio investors, a source familiar with the matter told Reuters.
Analysts said Vedanta's stake purchase in Cairn India would be useful if it failed to get a big response from minority shareholders for its open offer.
"This is a strategic move by Vedanta, and it makes the open offer inconsequential," said Jagannadham Thunuguntla, head of research at brokerage SMC Global Securities.
"Vedanta will now be able to get a comfortable controlling stake in Cairn India even if the open offer response is very poor," he said. "But all this is subjective to the government clearing the deal, and that is an unknown factor."
Cairn India did not respond to calls seeking comment on the block deal. Petronas confirmed it had sold its stake.
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"What this says is (Vedanta) are serious about the deal. They are putting their money where their mouth is, signalling intent and perhaps subtly turning up the pressure (on the Indian government)," a London-based analyst told Reuters.
Vedanta's deal to buy a majority stake in India's No. 4 oil and gas firm has been caught up in a dispute over royalty payments, and the two sides have been waiting for government approval for eight months.
The offer by Sesa Goa to buy up to a 20 percent stake in Cairn India was launched on April 11 and closes on April 30.
Thunuguntla said many investors would not tender their shares because the gap between the open offer price and the market price had narrowed significantly.
Cairn India shares rose as much as 3 percent after the block deal on Tuesday to touch 346.15 rupees.
At 1330 GMT, Cairn Energy shares were up 1.7 percent in a FTSE index up 0.7 percent. Vedanta shares were up 1.8 percent.
Cairn India saw 283 million shares, or 14.9 percent of its share capital, changing hands in block share market deals on the Bombay Stock Exchange on Tuesday.
Vedanta will join BHP Billiton as the only miners with large oil interests.
It has said Cairn India has the potential to almost double current production to about 240,000 barrels of oil per day -- around a quarter of India's output -- allowing it to benefit from rising demand spurred by industrialisation, economic growth and an expanding population.
Bank of America Merrill Lynch represented Petronas in the deal.
Vedanta's deal to buy Cairn's India assets, which would be the biggest acquisition in the Indian oil sector, is widely seen as a litmus test for foreign investment into India.
Vedanta and Cairn Energy have extended the deadline to seal the deal to May 20 after they failed to get the approval from the Indian government within the previous deadline of April 15. ($1 = 44.6 rupees) (Additional reporting by Prashant Mehra and Clara Ferreira-Marques; Editing by Lincoln Feast and Will Waterman)