The state-backed Dagong Global Credit Rating Co. on Tuesday downgraded its rating on the United States to A-plus from double-A, maintaining its negative outlook.

It warned that the Federal Reserve’s plan to buy $600 billion of Treasury securities over eight months in its second go at so-called quantitative easing could trigger a creditor crisis.

“The new round of quantitative easing monetary policy adopted by the Federal Reserve has brought about an obvious trend of depreciation of the U.S. dollar, and the continuation and deepening of credit crisis in the U.S.,” Dagong writes in its latest report on its U.S. rating. “Such a move entirely encroaches on the interests of the creditors, indicating the decline of the U.S. government’s intention of debt repayment.”

This is a salient point because China is America’s biggest foreign creditor, with official Treasury holdings creeping up on the $1 trillion mark and unofficial holdings expanding rapidly as well.

The United States and its trading partners have had a disagreement or two in recent days over Fed chief Ben Bernanke’s decision to expand his holdings of Treasurys in a bid to further bring down interest rates. Lower rates tend to equal a lower dollar, which tragically enough mucks up everyone else’s plans to export their way to prosperity.

One critique of the Fed’s decision is that it won’t actually accomplish what the Fed is setting out to do, which is to bolster domestic employment. Meanwhile, many critics contend, additional money-creation is apt to inflate asset bubbles elsewhere and eventually end in ruinous inflation for everyone.

But Dagong is willing to go where many observers have been unwilling to tread, contending that the collapse of the dollar is near.

“Analysis shows that the crisis confronting the U.S. cannot be ultimately resolved through currency depreciation,” it writes. “On the contrary, it is likely that an overall crisis might be triggered by the U.S. government’s policy to continuously depreciate the U.S. dollar against the will of creditors.”

Of course, analysis also shows that the supposedly debilitating depreciation of the dollar hasn’t gone as far as many would suppose.

Yes, the dollar has fallen by double-digit percentages in recent months against the yen and the euro. But the Federal Reserve’s broad trade-weighted exchange index, which tracks the dollar’s value against smaller currencies as well, is down less sharply. What’s more, it is flat with its level three years ago, before the onset of the credit crisis.

There is also the small matter of Dagong’s evident flair for drama. For instance, when it started coverage of the U.S. and other big debt-addled nations in July, it gave them lower ratings than the beloved Moody’s and S&P agencies in the U.S. in part because “Dagong does not to apply ideology as demarcation and fairly maintains interests of various circles in the national credit relationship.”

But that high-mindedness was in short supply as the company pursued its application for favored status with the Securities and Exchange Commission.

The SEC eventually turned down Dagong on the reasonable grounds it doesn’t have offices or clients in this country — but not until Dagong first claimed prematurely it had been rejected and then fired off a letter threatening a lawsuit and warning that China “shall share the discourse power of the credit rating in the U.S. market.”

We’ll see about that yet, but for now Dagong is saying the U.S. “may face … unpredictable risks in solvency in the coming one to two years.” Sad to say that even considering the source, that doesn’t seem terribly far-fetched.