* Asian stock markets : tmsnrt.rs/2zpUAr4 * S&P 500 futures flat as U.S. relief bill agreed * New strain of coronavirus shuts much of UK * Pound slips as Brexit talks drag on with no deal SYDNEY, Dec 21 (Reuters) - U.S. stock futures traded sideways in Asia on Monday as investors gave a cautious welcome to news a deal had been struck on a long-awaited U.S. stimulus bill, though “difficult” Brexit talks dragged on with no agreement in sight. Sterling slipped 0.6% to $1.3436 after several European countries closed their borders to the UK as the country entered a tougher lockdown to fight a new strain of coronavirus. Prime Minister Boris Johnson will chair an emergency response meeting on Monday to discuss international travel and the flow of freight in and out of Britain. In the United States, Republican U.S. Senate Majority Leader Mitch McConnell said an agreement had been reached by congressional leaders on a roughly $900 billion COVID-19 relief bill. The news saw futures for the S&P 500 jump at first, only to fade back to flat as the morning progressed. Nikkei futures were trading firmer at 26,885, after the cash index closed at 26,763 on Friday. Analysts at BofA noted a huge $46.4 billion flowed into equities in the latest week, while the outflow from cash was the largest in four months. There were record flows into tech shares, a large flows to the consumer sector, healthcare, financials, real estate and value stocks. BofA chief investment strategist Michael Hartnett said a “sell signal” had been triggered for the first time since February as cash levels declined to 4.0% in the latest Global Fund Manager Survey. “Positioning is getting over-extended as policy support and profits are peaking,” he said in a note. “Expectations for higher growth, inflation and lower interest rates have become consensus and investors are positioning for a very rosy scenario of low volatility and high growth.” Another popular trade has been shorting the U.S. dollar and again positioning was looking overextended by many measures, giving the currency some respite on Monday. The dollar index edged up a little to 90.147 and away from last week’s trough of 89.723 which had been the lowest since April 2018. The euro likewise edged back to $1.2230, while the dollar was a shade firmer on the yen at 103.36. “FX markets await final outcomes of a possible Brexit deal and U.S. fiscal package,” said Ned Rumpeltin, European head of FX strategy at TD Securities. “We remain biased to fade any ‘good news’ kneejerk USD-selling on both fronts, however. These factors look fully priced and the short-USD trade appears increasingly crowded.” The pause in the dollar’s decline saw gold prices pare a little of their recent gains, dipping 0.3% to $1,880 an ounce .