BRUSSELS — European Union leaders threatened the United States and China with trade sanctions Friday if the world's two biggest polluters don't commit to ambitious cuts in greenhouse gases by next year.

The warning came as the economic downturn focused European leaders on the impact on industry of their groundbreaking agreement last year to cut carbon emissions by 20 percent from 1990 levels by 2020.

EU leaders want similar commitments from other major economies by next year, when a conference on global warning is to take place in Copenhagen, Denmark. Otherwise they say European companies will need protection from unfair competition from heavily polluting rivals in China and the United States — the world's biggest emitters of carbon dioxide.

In a declaration issued after a two-day summit, the 27 EU leaders warned: "If international negotiations fail, appropriate measures can be taken" to protect European industry.

French President Nicolas Sarkozy went further. "Our main concern is to set up a mechanism that would allow us to strike against the imports of countries that don't play by the rules of the game on environmental protection," he told reporters Friday.

German Chancellor Angela Merkel noted the threat to European producers. "Industry, faced with global competition could be exposed to a real disadvantage if no international climate accord is struck, but we in Europe have very strict rules," she said.

Still, Germany and Britain were wary about the threat of sanctions, fearing that waving a big stick at the Chinese and Americans could make it harder to strike a compromise.

"I don't think we should allow things to stand in the way of getting the best possible international agreement," cautioned British Prime Minister Gordon Brown.

As an alternative, Germany wants an agreement within Europe that would exempt heavily polluting industries such as steel and cement from the cuts demanded by the EU plan, which are estimated to cost industry $78 billion.

However, the idea of protecting polluters drew condemnation from environmentalists.

"European leaders continue to focus on the dinosaurs in the energy- intensive industry," said Stephan Singer, head of WWF. "They run the risk to miss the big picture of the huge costs Europe and humanity will have to face if no serious action is taken now."

Despite the economic concerns, the EU leaders said they would stick to their carbon-cutting targets, which are among the world's most ambitious. They set an end-of-year deadline for resolving differences among the 27-nation bloc over which countries will have to bear the biggest burdens of the cuts.