U.S. Treasurys have rallied in recent days as worries about slowing growth have overtaken concerns about the sustainability of U.S. government debt. But one analyst says Treasurys are among the riskiest assets on the planet today and investors should look at Asian government bonds instead.

"I'm afraid Fitch and Moody's haven't exactly covered themselves in glory by maintaining the triple A rating," Don Amstad, Director of Asian Fixed Income at Aberdeen Asset Management told CNBC on Wednesday.