TOKYO—Group of Seven (G-7) nations sought to head off a collapse in investor confidence after the US sovereign-rating cut and a slump in Italian and Spanish debt intensified threats to the global economy.

G-7 finance ministers and central bank governors pledged in a statement to “take all necessary measures to support financial stability and growth.” Officials will inject liquidity and act against disorderly currency moves as needed, they said after a call late Sunday.

Stocks extended declines that have wiped $5.4 trillion off equity markets since July 26, driven investors to Treasuries and gold and rattled consumer confidence already hurt by European fiscal tightening and elevated American unemployment. The European Central Bank (ECB) signaled it will buy Italian and Spanish bonds, and Japan warned it may intervene again to stem yen gains.

“The markets can’t see a light at the end of tunnel” even after the G-7 statement because the pledge of cooperation isn’t enough, said Atsushi Ito, a senior rate strategist in Tokyo at UBS AG, who previously worked at Japan’s finance ministry. “Each nation has to come up with specific and convincing measures but it will probably take a while.”

The MSCI Asia Pacific Index of shares fell 2.6 percent as of 12:40 p.m. in Tokyo, heading for the worst five-day rout since October 2008. Futures contracts on the US Standard & Poor’s (S&P) 500 Stock Index lost 2.2 percent. The dollar reached an all-time low of 74.85 Swiss centimes before trading at 76.02. Gold pierced $1,700 an ounce in intraday trading for the first time.

Treasuries were little changed, with benchmark 10-year yields at 2.54 percent, after trading opened for the first time since the S&P announcement of the one-step cut in the US sovereign rating to AA+.

G-7 policy-makers committed to “coordinated action where needed,” and to “consult closely in regard to actions in exchange markets,” even as they reiterated support for market-set currencies. The group last mounted a joint foreign-exchange intervention in March, countering a surge in the yen after Japan’s earthquake.

While the G-7 statement was “better than nothing,” it’s not likely to stem the dollar’s decline, said Hiroaki Muto, a senior economist at Sumitomo Mitsui Asset Management Co. in Tokyo. Muto added that the scope for coordinated policy action is limited by inflation pressures, caps on spending, and interest rates that are already near zero in nations such as the US and Japan.

The yen rose 0.3 percent to 78.17 per dollar, about 2.5 percent from the postwar high reached in March. Investors flocked to the yen and franc amid the turmoil as currencies of nations with current-account surpluses. The yen’s gain risked damping a recovery from Japan’s estimated three quarters of economic contraction through June, by hurting exports.

Avoiding another “severe” recession may be “mission impossible,” Nouriel Roubini, co-founder and chairman of Roubini Global Economics Llc., wrote in the Financial Times.

In Europe, the debt crisis has seen yields on Italy’s 10-year government securities rising to 6.1 percent from 4.6 percent in two months, while Spain’s yields have soared to as high as 6.29 percent, from 5.14 percent in March. Ten-year borrowing rates for both nations have reached the most since before the euro was introduced in 1999.

“No change in fundamentals warrants the recent financial tensions faced by Spain and Italy,” said the G-7, made up of the US, Canada, UK, Germany, France, Italy and Japan. Policy measures announced by Italy and Spain will “strengthen fiscal discipline and underpin the recovery in economic activity and job creation,” the officials said.

The statement added that “the involvement of the private sector in Greece is an extraordinary measure due to unique circumstances that will not be applied to any other member states of the euro area.” The latest bailout package for Greece includes voluntary contributions from private-sector bondholders.

ECB said it will “actively implement” its bond-purchase program, in a statement issued in the name of President Jean-Claude Trichet after an emergency teleconference meeting of policy-makers, separate from the G-7 call.

The bank also called on all euro-area governments to follow through on the measures agreed in July, including allowing the European Financial Stability Facility to buy bonds on the secondary market.

“It is on the basis of the above assessments that the ECB will actively implement its Securities Markets Program,” the central bank said.

After a series of calls involving German Chancellor Angela Merkel, French President Nicolas Sarkozy, Spanish Prime Minister Jose Luis Rodriguez Zapatero and Italian Prime Minister Silvio Berlusconi, European leaders pledged to push implementation of last month’s deal on the European Financial Stability Facility. Berlusconi announced measures to speed austerity and target a balanced budget in 2013, a year ahead of schedule.

“The ECB is now in for the long haul and will potentially have to buy up to half of the Italian and Spanish traded debt, the biggest risk-pulling effort ever engineered in Europe,” said Jacques Cailloux, chief European economist at Royal Bank of Scotland in London.

Sovereign investors from Europe to Asia expressed confidence in US government debt after the S&P
decision.

Japan, the second-largest international investor in American government debt, sees no problem with trust in the securities, a Japanese government official said on condition of anonymity two days ago after the S&P decision.

Japan’s efforts to weaken the yen may boost its need to purchase Treasuries, after the nation’s authorities mounted the third round of yen sales since September last week. Vice Finance Minister Fumihiko Igarashi said over the weekend that the government is ready to intervene again.

Russia considers US debt reliable and won’t review its policy of investing in the country, Deputy Finance Minister Sergei Storchak said after the US downgrade. The cut “can be ignored” for long-term investment strategy, he said. Russia, a Group of 20 member, is one of the 10 largest foreign holders of US government debt.