Standard & Poor's issued a harsh warning Wednesday on Japan's economy, downgrading the nation's credit rating and putting its biggest banks on credit watch, and said the prime minister's promised economic reforms are happening too slowly.

The ratings agency called for "radical action" to help the economy grow, saying Japan is plagued by a crippled financial sector, rising debt burden and "fundamental institutional dysfunction."

Finance Minister Masajuro Shiokawa tried to downplay S&P's move, saying, "I think we need to take it seriously but I have confidence in our bonds."

Prime Minister Junichiro Koizumi has promised reforms to clean up the massive bad debts at Japanese banks, rein in public spending and turn over money-losing public businesses to the private sector. Still, there are signs that Japan's decade-long slump may be deepening.

S&P cut Japan's sovereign credit rating on Wednesday to AA from AA+. It maintained its rating for the outlook on Japanese credit -- which stands at "negative," two months after being downgraded from "stable."

The banks placed on review for a possible downgrade include the four that make up the Mizuho Group, the world's largest private banking institution.

S&P's assessment came two days after international credit agency Fitch Ltd. cut Japan's debt rating, saying recent government public-spending programs had dragged Japan deeper into debt.

Takahira Ogawa, director of sovereign ratings at S&P Asia, said the custodians of Japan's economy are failing to grasp the magnitude of the crisis.

"One of the problems is that there doesn't appear to be a sense of urgency or crisis among key decision makers," he said.

The agency warned that the government will have to inject funds worth 3 percent of the nation's gross domestic product into the banking sector to keep it solvent.

Though Koizumi has promised to cut government spending, his government has drawn up a second supplementary budget worth $24.97 billion. S&P said the move pushing public finances deeper into the red is "unsustainable." That also will make it more difficult for the government to pay off its public bonds, analysts said.

Japan's public debt currently stands at about $5.4 trillion, or 130 percent of gross domestic product.

The ratings agency also said necessary reforms are hindered because the government is beholden to agriculture, construction and retail interest groups.

"The natural conservative nature of a bureaucracy also mitigates against reform," S&P said.

Analysts, however, said the downgrade could help the prime minister push through his reform agenda, which includes curbing public spending.

"It reinforces Koizumi's message that Japan needs to come to grips with its fiscal situation," said Matthew Poggi, an economist at Lehman Brothers in Tokyo.

Wednesday's announcements helped push Tokyo's benchmark Nikkei stock index down nearly 3 percent.

S&P's moves came weeks after the government changed its economic growth forecast to negative 0.9 percent for the fiscal year ending in March from 1.7 percent growth. Unemployment rose to a record 5.3 percent in September.