BEIJING, April 20 (Reuters) - London aluminium and nickel prices fell for a second day on Friday as a rally driven by fears of supply disruptions caused by U.S. sanctions on Russia's United Company Rusal, the world's second-biggest aluminium producer, lost momentum. Prices have started to retreat amid no signs of further sanctions yet, brokerage Sucden said in a note, adding that conditions remain "nervous and choppy amid the overhanging uncertainty." Short-term direction was "likely to be headline-driven," it added. Aluminium is on track for a weekly rise of 6.3 percent in London and is up 21 percent so far this month. It climbed 11.9 percent last week in the wake of the Rusal sanctions announced on April 6, its biggest weekly jump since 1988. FUNDAMENTALS * LME ALUMINIUM: Three-month aluminium on the London Metal Exchange was down 2.1 percent at $2,432.50 a tonne by 0439 GMT, after ending down 2.1 percent in the previous session. Earlier on Thursday, it rose to $2,718, the most since May 5, 2011. * SHFE ALUMINIUM: The most-traded June aluminium contract on the Shanghai Futures Exchange was also down 2.1 percent by the mid-session interval at 14,965 yuan ($2,380.88) a tonne. * NICKEL: LME nickel was down 3.4 percent, extending its 1.3 percent fall on Thursday, when it rose to as much as $16,690, the most since Dec. 12, 2014, on fears the sanctions could be broadened to Russian nickel producer Nornickel . * SHFE NICKEL: The most-traded July nickel contract on the ShFE was down 3.3 percent to 104,180 yuan a tonne, after rising to its highest since June 2015 on Thursday. The exchange hiked transaction fees for the July contract from Friday. * ALUMINIUM: Rusal is stockpiling large quantities of aluminium at one of its plants in Siberia because U.S. sanctions imposed this month have prevented it from selling the metal to customers, five sources close to the company said. * BANKS: Lenders to Rusal are exploring how to get rid of their exposure before a May deadline set by the United States, as the fallout from fresh sanctions pulls Russian loan pricing lower in Europe's secondary market. For the top stories in metals and other news, click or MARKETS NEWS * Asian shares slipped on Friday as a warning on smartphone demand from the world's largest contract chipmaker slugged the tech sector, while high oil prices stirred inflation fears and undermined sovereign bonds.