* Purchases suspended indefinitely from Sept. 10
* Novatek shares down 5 pct
* Gas pipeline operator cites poor demand
* Rosneft, Novatek have been expanding aggressively on domestic market (Adds detail, analyst comments, stock market reaction)
By Denis Pinchuk and Melissa Akin
MOSCOW, Sept 10 (Reuters) - Russia's Gazprom, which has borne the brunt of weak demand in Europe and Russia, said it was suspending purchases of gas as of Monday from non-Gazprom producers, forcing them to share the pain of output cuts and ease a mounting supply glut.
"Today the gas market in Russia has an excess of resources over demand. In this situation we are looking at cuts to gas intake from independent producers," Gazprom's deputy head of marketing and liquids processing, Alexander Mikheyev, told a conference call on Monday.
Earlier, Gazprom said it had sent a letter to suppliers notifying them of the suspension.
Analysts said Gazprom was acting to preserve its own output by forcing so-called independents - oil companies and other suppliers outside the Gazprom group - to leave gas in the ground instead of pursuing ambitious growth plans at Gazprom's expense.
Domestic demand is down 3.6 percent so far this year, even after a slight recovery in recent months, said Karen Kostanian, an oil analyst at Merrill Lynch.
"In this environment, Gazprom would have to shut in its own production to sustain the independents," Kostanian said by telephone.
"In this natural gas environment, where Russia is declining, Europe is declining and China is not happening, it is only natural that the independents need to share the pain."
Gazprom is facing fierce pressure in its core European export market, where it earns the bulk of its revenues. European regulators said last week they would launch an anti-trust investigation into Gazprom's pricing after raids on its European units last year.
Its European sales fell 10 percent year-on-year in January-August despite concessions to key consumers, who enjoy a proliferation of alternative supplies from spot markets and seaborne liquefied natural gas.
Its only viable option for growth is to turn to Asia, but the lynchpin of its Asian strategy, a contract to sell gas to China, has eluded Gazprom in more than half a decade of talks which have been characterised by a wide standoff on pricing.
The suspension is a blow to mainly to Novatek, Russia's second largest gas producer, which has built its current business around domestic supply since Gazprom's export monopoly bars it from exporting gas.
LUKOIL, Russia's No. 2 oil company which is also the second largest seller of gas to Gazprom, said it had resolved the issue with the state gas monopoly. A Rosneft source said the company had not received the letter at all.
Novatek, controlled by Gunvor trading house co-founder Gennady Timchenko and CEO Leonid Mikhelson, declined to comment immediately.
Uralsib analysts said Gazprom's decision could affect 35 percent of Novatek's output, though it said it expected the suspension to be short lived, while Troika Dialog brokerage said Novatek's 2012 production growth target of between 7 and 11 percent is "under threat".
Last year, Gazprom's domestic sales totalled 307 billion cubic metres (bcm), of which 42 bcm were bought from the third parties.
"We confirm we got the letter and we are now discussing the situation with Gazprom. Until these talks are complete we will not be commenting on their details," a Novatek spokesman said.
In Moscow trade, Gazprom shares rose 0.6 percent to 164.21 roubles, while Novatek fell 2.52 percent to 360.60 roubles.
Gazprom did not specify the total volumes affected, but said the decision to suspend gas purchases from independents would concern purchases only by the parent company, not by other Gazprom group companies.
It would also not affect transportation of independents' gas supplies to their own customers through the Gazprom controlled unified gas supply system, it added.
Most of Russia's major oil producers, have large reserves and ambitious plans to increase gas production, which, as long as Gazprom's export monopoly is in place, are the source of a looming supply glut within Russia.
They have also gone direct to an increasing number of consumers to seal direct sales contracts at a time when Gazprom's domestic business is also looking vulnerable.
Russian media have reported that the bulk of Gazprom's supply contracts with domestic generators were five-year deals signed just before the final breakup of the state utility monopoly in 2007, and are due for renewal.
Independents are in a position to undercut Gazprom, which is largely bound to sell at regulated domestic prices, while rivals can set their own pricing policy and often have lower marginal production costs than Gazprom.
"As mature fields are sensitive to production cuts, we think the government will support Gazprom. We estimate domestic gas trading is less profitable for Gazprom as compared to own production," UBS analysts said in a research note.
"Therefore, we believe the cut will have a positive impact on earnings."
Reporting by Denis Pinchuk; writing by Melissa Akin; editing by Keiron Henderson