The recession-plagued US now faces higher interest rates throughout the market, possibly by as much as 10-15 basis points.
The rating change could have some bearing for New Zealand because the Reserve Bank may have to delay increasing its interest-driving official cash rate (OCR). A rise had been expected in September.
While historically low at 2.5%, New Zealand's OCR has been attracting overseas investors, which has been underpinning the New Zealand dollar strength and stymieing export profits.
Costlier US borrowing means weakened growth in the world's largest economy, but investor attention will now switch back to the European debt crisis and closer scrutiny of China's growth prospects.
On Friday (Saturday NZ time), international rating agency Standard & Poor's downgraded the US rating for the first time in its history from its coveted AAA to AA+ on negative outlook, with a warning another downgrade could be considered within 12 to 18 months. New Zealand's rating is also AA+.
"The downgrade reflects our opinion that the fiscal consolidation plan that Congress and the Administration recently agreed to falls short of what, in our view, would be necessary to stabilise the Government's medium-term debt dynamics," S&P said in a statement.
Employment data released in the US last Friday came in slightly better than expected and saw some stability return to stocks on Wall St, after Thursday's rout lowered key New York stock exchange indexes by between 3% and more than 5%.
Craigs Investment Partners broker Peter McIntyre said that with Australia and New Zealand being the first exchanges to open this morning, some "nervousness in the markets" would remain.
"Commodities have already been sold off and the mining sector will come under pressure as the outlook for global growth is weaker," he said yesterday.
The S&P US downgrade, it said it had expected deeper cuts than the $US2 trillion announced last week, was not unexpected and was "positive" in as much as markets would see the US addressing its immediate debt issues.
Mr McIntyre forecast the downgrade would add up to 15 basis points to US borrowing.
While the past week's market turmoil has been likened to the banking crisis about two years ago, Mr McIntyre highlighted that, unlike bank lending in 2008-09, which dried up, credit from banks was still flowing and the US economy was carrying on.
The US lost its top-tier rating hours after investors alarmed by the euro-zone debt crisis forced Italy to speed up an austerity drive.
China, the largest foreign holder of US government debt, made clear that Washington only had itself to blame, and called for a new stable global reserve currency.
In a sign of how concerned world leaders were about the costly $US2.5 trillion ($2.9 trillion) routing, Italian Prime Minister Silvio Berlusconi said finance ministers from the Group of Seven nations would meet in "just a few days" to seek a common plan of action.
No other member of the group - which does not include world No2 economy China - has confirmed the meeting.
Worries the euro-zone debt crisis was spreading and the United States was slipping into recession drove the week-long rout in financial markets.
Better-than-expected jobs growth in July helped support Wall Street on Friday but stocks slipped back into the red in late trading.
By calling the outlook "negative", S&P signalled another downgrade was possible in the next 12 to 18 months.
It blamed in part the political gridlock in Washington, saying politics was preventing the United States from addressing its deficit and debt problems, a view supported in Beijing.
"The US Government has to come to terms with the painful fact that the good old days when it could just borrow its way out of messes of its own making are finally gone," China's official Xinhua news agency wrote.
"International supervision over the issue of US dollars should be introduced and a new, stable and secured global reserve currency may also be an option to avert a catastrophe caused by any single country," it said.
- Additional reporting Reuters
