By Wojciech Moskwa and Richard Solem

OSLO (Reuters) - Norway's government said it has started to increase its stake in oil and gas producer StatoilHydro to solidify control over its biggest revenue earner, boosting the company's shares on Monday.

The government aims to raise its stake to 67 percent from 62.5 percent -- an investment of nearly $6 billion at present valuations -- by buying shares "in the market, over time", according to its parliamentary mandate.

Norway's shareholder registry showed the government's stake in StatoilHydro increased by 285,000 shares last week, a fraction of the 143 million shares it needs to buy to raise its stake by 4.5 percentage points.

Some analysts said it was surprising that Norway appeared to have bought the shares during trading hours, not in a special tender after market close.

"Some were expecting they would buy shares in an auction after market close but this time they only bought a very limited number of shares," said Carnegie analyst John Olaisen.

Shares in StatoilHydro, already up on high oil prices, rose as much as 3.8 percent on Monday on the news. By 0943 GMT (10:43 a.m. BST) they were 198.40 crowns, up 2 percent, against a 1.2 percent rise on the DJ Stoxx oil and gas index.

"I can confirm that the state has bought shares," said Unni Claussen, a spokeswoman for the Petroleum and Energy Ministry.

She declined to comment further on the transaction or the timing of such purchases in the future.