MANILA, April 18 (Reuters) - London aluminium futures rose to their strongest level since 2011 on Wednesday as growing worries over tighter supply following U.S. sanctions on key producer Rusal sustained a rally that has lifted the metal by 20 percent this month. Global miner Rio Tinto said adjustments may be made to its 2018 aluminium output following sanctions on its Russian partner UC Rusal imposed by the United States earlier this month. "The sanction against Rusal and resulting dissociation with Rusal by its banks, suppliers, customers and related parties resulted in heightened concern about a global supply shortage," said Helen Lau, analyst at Argonaut Securities. Three-month aluminium on the London Metal Exchange climbed as high as $2,445 a tonne in early Asian deals, its strongest since September 2011, before turning flat at $2,405 by 0321 GMT. There was little impact on base metals prices from China's surprise move to cut banks' reserve requirement ratio (RRR), which only lifted steel and coking coal futures sharply. "The RRR cut was positive, but it may not have very significant impact on financial markets. In terms of size, the steel market is also much bigger than base metals," said CRU analyst Wan Ling. China's central bank unexpectedly said it will reduce the cash banks hold as reserves by 100 basis points from April 25. But the move falls short of broad monetary easing, with the authority attaching requirements on how funds must be used. SHANGHAI ALUMINIUM: On the Shanghai Futures Exchange, the most-traded June aluminium contract was also nearly flat at 14,850 yuan ($2,362) a tonne, below a nearly three-month high reached on Tuesday. RIO TINTO: Rio last week declared force majeure on some customer contracts in light of the sanctions on Rusal, the world's second biggest aluminium producer, and said it is reviewing Rusal's 20 percent stake in its Queensland Alumina refinery. RUSAL HALTS ORDERS: Rusal has stopped placing orders with two logistics firms that ship its exports, in a sign of deepening problems for the company. DIVESTMENT: Rusal may be forced to divest most of its portfolio of overseas operations if the aluminium giant cannot restructure them to evade the U.S. sanctions and ensure a flow of raw materials. JAPAN TRADERS: Major Japanese trading houses have asked Rusal to stop shipping refined aluminium and other products and are scrambling to secure metal elsewhere, industry sources said. CHINA OUTPUT: Amid the prospect of tighter supply elsewhere, China's aluminium production rose 4 percent from a year ago to 2.78 million tonnes in March as supply from new smelters outweighed winter output curbs imposed on existing producers. CHINA RRR CUT: The People's Bank of China said banks must use most of the freed-up liquidity to pay back relatively costly loans obtained via the central bank's medium-term lending facility (MLF). Based on first-quarter data, the PBOC said the MLF loans due to be repaid on April 25 will be about 900 billion yuan ($143 billion). ALUMINIUM SHEET: The U.S. Commerce Department said it has made a preliminary determination that aluminium sheet imports from China are being subsidized. COPPER: Benchmark LME copper slipped 0.2 percent to $6,865.50 a tonne. In Shanghai, copper dropped 0.3 percent to 50,710 yuan. MARKETS: Asian shares edged higher, tracking gains on Wall Street, although Chinese equities struggled even as Beijing boosted liquidity in the banking system. The dollar clung to modest gains.