Amgen Inc.’s venture capital arm has partnered with two other firms to invest in biomedical startups through a new $265 million fund. The Thousand Oaks company’s Amgen Ventures is a limited partner in the joint venture, led by Cambridge, Mass. venture capital firm Atlas Venture. Swiss biopharmaceutical company Novartis AG also is a partner in the fund. Both Novartis and Amgen have facilities in the Cambridge area, and will work with Atlas to find life-sciences startups to fund. “Amgen is very excited to partner with Atlas Venture, one of a small number of venture capital groups that truly focuses on creating value by translating groundbreaking science into new companies,” said Sean E. Harper, executive vice president of research and development at Amgen, in a statement. Atlas will lead the efforts, and neither Amgen nor Novartis have exclusivity rights or required commitments to pursue development of any drugs produced by the startups, Atlas noted in a statement. Amgen Ventures has stepped up its interest in backing or partnering to fund several startups in recent months. In April it was one of several companies to contribute to Series C funding of Opsona Therapeutics Ltd., a Dublin, Ireland immune drug development company. In November, the firm announced that it was backing life-sciences company Atara Biotherapeutics Inc. jointly with Silicon Valley venture capital firm Kleiner Perkins Caufield & Byers.