* FTSE 100 up 0.1 percent, just shy of 6,000 level
* Energy stocks gain as crude price gains
* Rio Tinto slips after making Riversdale offer
By Simon Falush
LONDON, Dec 23 (Reuters) - A stronger energy sector fuelled slight gains in Britain's FTSE 100 share index early on Thursday in light trade ahead of the Christmas holiday, but some slight falls in banks and miners kept the index below the 6,000 level.
By 0847 GMT the FTSE 100 was 5.19 points or 0.1 percent higher at 5,988.68, off an earlier high of 5,999.16.
The blue-chip index is at its highest since July 2008, has gained 10.6 percent this year and is up 8.3 percent in December, its biggest monthly gain since August 2009.
"The line of least resistance seems to be up," Andrew Bell, chief executive of investment trust Witan said.
"The EU debt crisis mark 2 has not hit investor sentiment as much as the first one did in May and economic growth seems to be chugging along OK."
Energy stocks gave the most support to the index, lifted as crude climbed towards a two-year high. BP added 0.7 percent while Royal Dutch Shell gained 0.3 percent.
If the psychologically important 6,000 level is surpassed, investors will be looking at technical resistance levels to assess how fast the index may rise further.
Michael Hewson, strategist at CMC Markets said the next strong resistance is at 6,075.
"FTSE needs to close the week above 5,976 which is the 76.4 percent Fibonacci retracement of the down move from 6,754 to the lows in 2009 at 3,460 to target these highs."
Banks were the main drag on the index, retreating slightly after three days of gains.
Miners were slightly weaker, with Rio Tinto 0.6 percent lower after the Anglo-Australian miner offered $3.9 billion to buy African-focused coal miner Riversdale in an agreed deal that is expected to be challenged by rivals.
Looking at the bigger picture, the Bank of England's executive director for markets Paul Fisher said in a newspaper interview on Thursday that Britain's economy could suffer another period of contraction in 2011.
U.S. economic data out on Thursday includes November durable goods data and the latest weekly jobless claims data, both due at 1330 GMT, with the final reading of the December Reuters/University of Michigan consumer sentiment survey out at 1455 GMT and November new home sales data scheduled for release at 1500 GMT. (Editing by Greg Mahlich)