By Marius Zaharia LONDON (Reuters) - German 10-year borrowing costs resumed a descent towards zero on Monday, with worries about Greece falling out of the euro zone increasing demand for top-rated assets even as the ECB's bond-buying programme quashes yields. Athens has been stuck in negotiations with its euro zone partners and the International Monetary Fund over economic reforms required by its lenders to unlock remaining bailout funds as it is rapidly running out of cash. The IMF's European head, Poul Thomsen, told German newspaper Handelsblatt talks have gained momentum but they were still a long way from the finish line. Greece's Finance Minister Yanis Varoufakis said on Sunday that if Athens were to leave the euro zone, there would be an inevitable contagion effect.