Most of the commodities moving across the U.S. border with Canada did so through pipelines, the U.S. Department of Transportation said.

The Department of Transportation published North American freight numbers for May. DOT said trade included under the U.S.-North American Free Trade Agreement totaled $103.9 billion and transit increased for rail, trucks, pipelines and maritime vessels.

For pipelines, the department said the value of U.S.-Canadian trade increased the most when compared with other modes of transit.

Canada is the top crude oil exporter to the United States. The U.S. Energy Information Administration said an average 2.7 million barrels of oil per day was imported from Canada during the week ending July 18, a 6.6 percent increase year-on-year.

"U.S.-Canada pipeline trade comprised 94.8 percent of total U.S.-NAFTA pipeline trade in May," the department said Thursday.

Rail was the second-largest mode, moving about 15 percent of all traded commodities in North America. The glut of oil produced in North America has outpaced pipeline capacity, a trend those in the energy industry says leaves rail as the primary alternative shipping method for crude oil.

The Department of Transportation this week issued a series of proposals on rail safety. Crude oil transport by rail has become a growing safety concern in the wake of last year's deadly derailment in Lac-Megantic, Quebec.