Meeting in Mexico City one day before the U.S. elections, the G-20 finance ministers issued a statement saying the United States faces "a potential sharp fiscal tightening."
"The United States will carefully calibrate the pace of fiscal tightening to ensure that public finances are placed on a sustainable long-run path, while avoiding a sharp fiscal contraction in 2013," the G-20 said in a statement.
Other delegates at the meeting expressed similar concerns.
"Whoever is going to be elected or re-elected tomorrow (in the United States) will be faced with that challenge, and will have to tackle that issue upfront, very shortly," said International Monetary Fund Managing Director Christine Lagarde.
"First and foremost the U.S. leadership needs to address quickly the so-called fiscal cliff and the debt ceiling, those two risks ... are clearly factors of uncertainty, not only for the U.S. economy but also for the global economy."
Agustin Carstens, the governor of Mexico's central bank, said the G-20 countries told the United States how important the issue was for continuing the world economic recovery.
While much of the attention at the two-day meeting focused on Europe's continuing financial crisis, E.U. officials were focusing the heat on the U.S. and other problems.
"The risks have decreased dramatically in the European area," said Olli Rehn, the EU's financial and monetary affairs commissioner. "There is agreement that solving the Euro-area crisis won't be enough for the world economy to have higher growth ... risks do also stem from the U.S. fiscal cliff, the high level of commodity prices, and the slowdown in emerging economies."
Despite the challenges, the G-20 statement said: "We are firmly committed to open trade and investment, expanding markets and resisting protectionism in all its forms."
In apparent reference to concerns that China or other countries might seek to combat a downturn in growth by manipulating currencies, the G-20 officials wrote, "we reiterate our commitments to move more rapidly toward more market-determined exchange rate systems and exchange rate flexibility to reflect underlying fundamentals, avoid persistent exchange rate misalignments and refrain from competitive devaluation of currencies."
Earlier Monday, Germany and the United Kingdom proposed that the world's biggest economies form a common front against tax evasion related to internet commerce and other revenue-shifting schemes, and said they received strong support at the meeting of officials from the G-20 nations.
"We've just been discussing it in the meeting we had. There was widespread support," U.K. Treasury chief George Osborne told reporters.
Osborne and German Finance Minister Wolfgang Schauble said they don't want to scare businesses away, but said companies must pay what they owe.
"International tax standards have had difficulty keeping up with changes in global business practices, such as the development of e-commerce," the two said in a joint statement. "As a result, some multinational businesses are able to shift the taxation of their profits away from the jurisdictions where they are being generated."
They said a united approach among the world's largest economy is the best way to fight evasion, without penalizing any single country.
"It's very important that we as individual countries don't price ourselves out of the world economy," Osborne said.
They did not mention which specific accounting procedures might be targeted or what enforcement measures were proposed, but Osborne said the goal was "acting together as the world's largest economies to make sure that international tax standards keep pace with international business."
They said the proposal had been forwarded to the Organization for Economic Cooperation and Development for study, and that team would report back to the Group of 20 finance ministers' next meeting, to be held in Moscow in February. Both ministers said they also support the OECD's own "tax-base erosion and profit-shifting" initiative, focused on the same problem.
"What we are doing today is starting a process," Osborne said.
The European financial crisis and the U.S. deficit had dominated the G-20 agenda.
On Monday, Spanish Economy Minister Luis de Guindos said none of the group's ministers had pressured his country to ask for a bailout package, which could come with onerous conditions, and he said Spain wouldn't accept any such pressure.
Despite weakness in its banking sector and pressure on government-bond interest rates, De Guindos said Spain "has relatively good liquidity" to see out the year.
Spanish Prime Minister Mariano Rajoy said last week that he saw no immediate need to ask for help, but did not rule it out in the future.
The two-day closed-door meeting comes just ahead of U.S. elections and lacks key players such as U.S. Treasury Secretary Timothy Geithner.
Mexican Treasury Secretary Jose Antonio Meade said over the weekend that the ministers plan to discuss "the fiscal cliff" in the United States, where a package of spending cuts and tax increases is set to take effect unless Congress acts by Jan. 1.
The G-20 brings together the world's principal economies and important emerging ones, including the United States, the European Union, China and Brazil.