* Copper to avg $7,100 in Q4 on supply shortfall-Barclays
* Rusal eyes record Q1 Japan aluminium premiums with $270 offer
* Coming Up: U.S. pending home sales at 1500 GMT
(Adds analyst's comments, updates prices)
SINGAPORE, Nov 25 (Reuters) - London copper slipped on Monday, snapping three days of gains, on growing worries the Federal Reserve could start paring its commodities-friendly monetary stimulus sooner than later in the face of an improving U.S. economy.
But forecasts for a shortfall in supply in top consumer China underpinned prices.
"Tapering is a big concern, that's for sure," said Sijin Cheng, analyst at Barclays in Singapore.
Tighter monetary policy going forward would restrict access to funds for business and commodity investors.
Investors will also look at a raft of housing reports from the United States to gauge the country's economic outlook. The Fed's last policy meeting suggested officials are preparing to reduce the pace of bond-buying in coming months as long as the economy continues to improve.
But trading is expected to quieten down this week ahead of the Thanksgiving holiday in the United States on Thursday.
Three-month copper on the London Metal Exchange edged down 0.24 percent to $7,078 a tonne by 0300 GMT. LME copper rose 1.2 percent last week, its biggest weekly gain in two months. But prices are down 2.3 percent so far in November and are poised for their biggest monthly drop since June.
The most-traded February copper contract on the Shanghai Futures Exchange was up 0.3 percent at 50,700 yuan ($8,300) a tonne, but had trimmed initial gains of around 1 percent.
Hedge funds and money managers cut bullish bets in futures and options of U.S. gold and silver, but increased copper's net short for the week ended Nov. 19, a report by the Commodity Futures Trading Commission showed on Friday.
"That's probably to do more with macro short positioning than discretionary money... which doesn't appear to be trading right now," Cheng of Barclays added.
A surfeit of mine supply is failing to boost refined copper stocks, partly because of lower output at some producers, and less availability of alternative feed stock scrap metal.
Reflecting a lack of supply of refined copper, cash prices remain at a premium to three month prices, although the premium eased by $1 to $3, data <CMCU0-3> showed on Friday.
Barclays sees a supply shortfall supporting copper prices in the fourth quarter with an average price of $7,100 a tonne.
China's importers of refined copper face a shortfall of about 30,000 tonnes this month and the next as a typhoon-hit Philippine smelter has delayed metal deliveries, supporting spot premiums near 4-year highs.
Supply of refined metal may be pressured out until next year. Top copper producer Chile's Codelco asked Chinese customers to accept a cut in term shipments in the first half of 2014, as it hiked fees for its metal by 41 percent to a nine-year high.
"People expect the tightness in copper to continue for the next two months," Cheng added.
In other metals, Russia's United Company Rusal Plc , the world's biggest aluminium producer, said it had offered January-March shipments of the metal to Japanese buyers at a premium of $270 per tonne, seeking a record high.
Three month LME copper
Most active ShFE copper
Three month LME aluminium
Most active ShFE aluminium
Three month LME zinc
Most active ShFE zinc
Three month LME lead
Most active ShFE lead
Three month LME nickel
Three month LME tin ($1 = 6.0936 Chinese yuan)
(Editing by Himani Sarkar and Muralikumar Anantharaman)