DHS head John Kelly gave a clear signal that the Trump administration is heading for a marijuana crackdown

Andrew Harnik/AP

Homeland Security Secretary John Kelly.

Secretary of Homeland Security John Kelly called marijuana a 'gateway drug' on Tuesday, a clear signal that the DHS's stance on marijuana reflects that of the Jeff Sessions-led Justice Department, and the Trump administration.

"Let me be clear about marijuana," Kelly said during comments at George Washington University in D.C. "It is a potentially dangerous gateway drug that frequently leads to the use of harder drugs."

Kelly referred to the "vast tonnages" of marijuana and hard drugs that 'TCOs' - transnational criminal organizations - move across the border from Mexico.

Though Kelly has come out in favor of prosecuting marijuana, he told NBC's "Meet the Press" on Sunday that marijuana "isn't a factor" in the drug war, and is scarcely the biggest issue on the US-Mexico border.

"The solution is not arresting a lot of users," Kelly added.

A number of states voted to legalize adult marijuana use in November, though it's still illegal at the federal level.

Kelly said that until Congress passes a law to change marijuana's federal status, "we in DHS are sworn to uphold all the laws on the books."

"Additionally, science tells us that it is not only psychologically addictive but can also have profound negative impact on the still developing brains of teens and up through the early 20s," Kelly said.

Kelly pledged to investigate marijuana's "illegal pathways" into the US, and arrest those involved with the drug trade "according to federal law."

Perhaps most tellingly, Kelly said that Immigration and Customs Enforcement, or ICE, would continue to use marijuana possession and distribution convictions as "essential elements" for deportation of undocumented immigrants.

Trump signed an executive order in February that vastly expands the powers of ICE and other border officials, which has resulted in an increase in the amount of undocumented immigrants arrested during the first few months of 2017.

The Trump administration's stance towards marijuana legalization has given states pause on whether to roll out new legislation, such as marijuana-friendly clubs and restaurants.