* FTSE 100 down 0.15 pct
* A number of shares trading ex-dividend
* CRH rises after H1 report (Adds closing prices)
By Julien Ponthus
LONDON, Aug 23 (Reuters) - UK shares gradually slid into negative territory on Thursday as trade tensions between China and the United States kept investors on their toes and a change of sentiment after the open on Wall Street weighed on shares.
The blue-chip FTSE 100 .FTSE index closed down 0.15 percent at 7,563.22 points with shares in the energy sector contributing to limit the losses.
Heavyweights Royal Dutch Shell RDSa.L and BP BP.L were up 0.8 percent and 0.5 percent respectively.
Smaller firms in the sector such as Tullow Oil TLW.L and Premier Oil PMO.L , which announced profits for the first half of the year, were also rising. Both were up about 1 percent.
"From a fundamental point of view, while growing fears for a trade war was a main story in the last few weeks, investors are now betting on crude again, due to the sharp drop in U.S. oil supply and the escalation of tension with Iran", said Carrlo Alberto De Casa, chief analyst at ActivTrades.
Miners and basic materials groups weighed on the index, with copper prices falling while the trade dispute between Washington and Beijing continued to trigger concerns about demand for industrial metals.
Anglo-American AAL.L and Fresnillo FRES.L lost 2.6 percent and 2.4 percent respectively.
"The fact the FTSE avoided a sharp drop after the bell is something of a surprise given the woeful state of the commodity sector this Thursday", commented Spreadex analyst Connor Campbell.
CRH CRH.L was up 0.7 percent after the building material firm published a trading update.
"Weather disruption has not prevented building materials business CRH from chalking up a solid first half", said AJ Bell investment director Russ Mould.
A number of stocks, notably in the financial sector, such as Prudential PRU.L , Royal Bank of Scotland RBS.L and the London Stock Exchange LSE.L were trading without entitlement to their latest dividend pay-out, trimming 4.24 points off the FTSE 100.