The Royal Bank of Scotland has launched the initial public offering of its US subsidiary Citizens Financial Group.Majority state-owned RBS plans to sell 25% or 140m shares in Citizens at between $23 and $25. RBS has also granted a 30-day over-allotment option of up to an extra 21m shares.Chief executive Ross McEwan said: "We announced our intention to undertake an IPO of Citizens in February 2013 and today’s launch is an important milestone for both RBS and Citizens."The planned divestment will significantly improve RBS's capital foundation and is a further important step in making RBS a strong and secure bank that continues to fully support the needs of its customers."RBS is off-loading non-core businesses as part of an overhaul drive to shore up its finances after the British taxpayer rescued it in the financial crisis, which resulted in the UK government owning about 81% of the bank.