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GUADALAJARA, Mexico (Reuters) – Leaders of the United States, Mexico and Canada vowed on Monday to fight the spread of the H1N1 swine flu and combat climate change but differed on trade disputes at their "three amigos" summit.

U.S. President Barack Obama, Mexican President Felipe Calderon and Canadian Prime Minister Stephen Harper met against a backdrop of an economic downturn in each country with a U.S. rebound key to a regional improvement.

Obama and Harper said their governments would share information as each faces the possibility of a predicted upsurge in the H1N1 virus this autumn.

"H1N1, as we know, will be back this winter," Calderon said at a joint news conference. "We are getting prepared, all three countries, to face in a responsible manner this contingency and abate its impacts for our people."

All three leaders vowed to respect the North American Free Trade Agreement that unites their countries in trade, but differed on some issues.

Harper raised with Obama Canada's concerns about the "Buy American" provisions in the $787 billion U.S. economic stimulus plan that the Canadians fear could shut out Canadian companies.

Canada is the United States' largest trading partner.

Obama said it was important to keep in perspective the fact that no sweeping protectionist measures have been imposed and that the "Buy American" provisions were limited to the stimulus and have "in no way endangered the billions of dollars in trade between our two countries."

Calderon, who is trying to persuade Obama to resolve a cross-border trucking dispute to allow Mexican trucks to transit into the United States, said all three leaders believe it is essential to abide by NAFTA and to "resolve the pending topics" impeding greater regional competitiveness.

Obama had made clear to Calderon that he was working with the U.S. Congress to resolve what he considers to be legitimate safety concerns with Mexican trucks.

He said the United States, Mexico and Canada should take steps to avoid protectionism, saying "we need to expand that trade, not restrict it."

The three leaders issued a statement on joint efforts to combat climate change with an eye toward a global summit on the topic in Copenhagen in November.

"We, the leaders of North American reaffirm the urgency and necessity of taking aggressive action on climate change," they said.