Keystone XL has been studied longer than any other project of its kind and it's time to start construction, advocates told U.S. Secretary of State John Kerry.

More than 40 industrial groups, ranging from the American Petroleum Institute to the Steel Manufacturers Association, sent a letter to Kerry in support of Keystone XL.

Pipeline company TransCanada submitted its application to the U.S. federal government to build a pipeline across the U.S.-Canadian border more than five years ago. It would connect to existing networks that feed refineries along the southern U.S. coast.

The letter to Kerry states that, since TransCanada's permit application was filed, more than 10,000 miles of oil and gas pipelines have been built in the United States.

"We once again request that the State Department grant the Keystone XL pipeline permit and put our labor forces back to work this construction season," their letter to Kerry read.

Supporters of Keystone XL say it would provide a source of economic stimulus and energy security. Detractors worry the heavier grade of oil it would carry is too great of an environmental threat to pursue.

The pipeline would pass through Nebraska, where a challenge over the pipeline's proposed route through the state is under way.

The U.S. State Department offered no comment on the letter to Kerry.