* Euro slips vs dollar, earlier climb loses momentum
* Investors trim short dollar positions but weakness intact
* Dlr/yen slides near pre-intervention levels, BOJ awaited
(Adds comment, updates throughout; previous TOKYO)
By Naomi Tajitsu
LONDON, Oct 4 (Reuters) - The euro slipped against the dollar as its inability to hold gains above a 6 1/2-month high hit earlier on Monday prompted investors to trim long positions in the single European currency.
The dollar gained slightly across the board, but it hovered in range of a 15-year low versus the yen on speculation the Federal Reserve could ease U.S. monetary policy further.
Analysts said the dollar may find some short-term relief as investors cover short positions in the currency on concerns that bets for further depreciation are piling up.
But many say the trend of dollar weakness will remain intact, although the prospect of more yen-selling intervention by Japan may limit its downside versus the Japanese currency.
"General dollar weakness can play out versus the euro, but not against the yen," said Ulrich Leuchtmann, currency analyst at Commerzbank in Frankfurt."As long as there's general dollar weakness and it trades so close to 83.00 yen, there's always the chance that Japan will step in."
He expected Japanese authorities to step in to sell yen if the dollar fell under 83 yen, as they did last month.
By 0754 GMT, the dollar traded at 83.20 yen, little changed on the day and pulling back 83.88 yen hit in Asian time.
The Bank of Japan began on Monday a two-day policy meeting and was expected to extend a cheap fund-supply tool to help shore up the struggling economy.
Some market participants said there was speculation any BOJ stimulus steps may be followed by more yen-selling intervention.
The euro slipped 0.4 percent to $1.3730, pulling back after earlier poking above $1.3800 for the first time since mid-March. Analysts said its rally lost momentum above that level, leading to short-covering in the dollar.
The euro had climbed early on Monday after China pledged on Sunday to support a stable euro and not reduce its holdings of European government bonds
But it was unable to hold those gains, and traders in London said Asian demand to sell euros was helping to push it lower, as was an early slide in European shares and M&A-related demand to sell the euro.
This helped push the dollar up against a currency basket, lifting the dollar index up 0.3 percent and pulling it back from 78.029 hit late last week, its weakest since January.
Still, few in the market believe the dollar's correction on Monday will continue, given the prospect of more quantitative easing by the Fed, which would likely increase dollar liquidity and lead to more currency weakness.
"Whether broad USD weakness will continue or not is one of the most important themes when we consider the trend in G10 currencies in coming weeks," JPMorgan analysts said in a note.
"We believe that it will remain the case as the market has not fully priced QE2 by the Fed."
Dollar-selling demand is mounting, with speculators' bets against the dollar swelling to $22 billion in the week to Sept. 28, the highest value since at least mid-2008, data from the Commodity Futures Trading Commission showed.
Long positions in the euro jumped to 35,330 contracts from 5,097 in the previous week, while long positions in the yen rose to 28,666 contracts from 23,100.
The Reserve Bank of Australia, the Bank of England and the European Central Bank also hold policy meetings this week.
U.S. non-farm payrolls data, due on Friday, will help markets gauge whether the labour market, seen as key for the country's economic recovery, is improving.