(Adds quotes from Trudeau letter, background)
OTTAWA, May 5 (Reuters) - Canada is considering a request to ban shipments of U.S. thermal coal through ports in the Pacific province of British Columbia, Prime Minister Justin Trudeau said on Friday, escalating a trade dispute with Washington.
Trudeau revealed the news in a letter to British Columbia premier Christy Clark, who asked him last month for the ban in response to fresh U.S. tariffs on Canadian exports of lumber. Her province is a major timber producer.
"The government of Canada is considering this request carefully and seriously. I have asked federal trade officials to further examine the request to inform our government's next steps," Trudeau said.
Clark, who faces an election on May 9, says she will apply a levy on thermal coal exports if she retains power and Trudeau does not act. A shortage of port capacity means some U.S. coal firms rely on Canada.
Trudeau's Liberal government says it will defend the lumber industry against what it calls an unfounded and unfair U.S. decision last month to impose the tariffs.
The dispute threatens to cloud talks on updating the North American Free Trade Agreement, which are set to start this year. NAFTA groups Canada, the United States and Mexico. (Reporting by David Ljunggren; Editing by James Dalgleish)