Thwarting cross-border security threats, coping with a potential bird flu outbreak and boosting North American trade loom large over ties between the United States, Canada and Mexico.

They top the agenda as Secretary of State Condoleezza Rice, Homeland Security Secretary Michael Chertoff and Commerce Secretary Carlos Gutierrez meet their Canadian and Mexican counterparts in Ottawa on Friday.

The officials were meeting as concerns mount that new U.S. border restrictions, imposed after the Sept. 11, 2001, terrorist attacks, may cripple thriving intra-continental flows of commerce and people.

With a combined gross domestic product of $15 trillion — overwhelmingly from the U.S. — the three nations exchange goods and services worth nearly $1 trillion and see about 500 million legal border crossings a year, U.S. statistics indicate.

The value of U.S. exports to Canada over a single bridge — the Ambassador linking Detroit and Windsor, Ontario — is greater than that of U.S. exports to Japan, they show.

Protecting such movements as well as the U.S., Canadian and Mexican populations from terrorism and pandemics are key mandates of the 2-year-old, three-nation Security and Prosperity Partnership.

Friday's meeting follows Chertoff's announcement Thursday that children will be exempt from new rules requiring travelers to show passports when entering the U.S. at land or sea borders.

The session is expected to produce recommendations on the matters for President Bush, Canadian Prime Minister Stephen Harper and Mexican President Felipe Calderon to consider at summit expected later this year in Canada.