WASHINGTON, Feb 10- The US call for NATO partners to step up funding for the transatlantic alliance is "a fair demand," German Defence Minister Ursula von der Leyen said on Friday after what she called a positive first meeting with Defense Secretary James Mattis.Germany and other European powers were unnerved when President Donald Trump during the election campaign accused NATO allies of failing to pay their way, and described the alliance shortly before he took office last month as "obsolete."Trump offered some reassurance this week when he told US forces: "We strongly support NATO."Von der Leyen said Germany, which spends less than the NATO target of 2 percent of economic output on defense, understood it needed to increase that amount."I think it's a fair demand," von der Leyen said. "If we want to jointly master the crises in the world, namely the fight against terrorism, and also put the alliance on solid footing, then everyone has to pay their share."