China's Foreign Ministry said on Tuesday that the United States must take "responsible" measures to protect investors in its debt after Standard & Poor's threatened to lower its credit rating on the United States due to a bulging budget deficit.

"We hope the U.S. government will take responsible policies and measures to safeguard investors' interests," the ministry said in a statement.

China's foreign exchange reserves, already the world's biggest, rose by nearly $200 billion in the first quarter to $3.05 trillion. About two-thirds are estimated to be invested in dollars.

Beijing has repeatedly warned that loose U.S. monetary policy threatens the dollar, but it has continued to accumulate dollar assets at the same time, adding about $260 billion of Treasury securities last year, according to U.S. data.