Members of the U.N.'s North Korea sanctions committee have seen media reports about the boat and are awaiting a formal notification with details from Panama.
"We are following it closely," said Jacques Flies, a spokesman for Luxembourg Ambassador Sylvie Lucas, who chairs the committee.
Authorities seized the vessel and the undeclared haul in the Panamanian port of Manzanillo.
U.S. officials say they tracked ship
Investigators spotted the boat going through the Panama Canal to Havana and then back toward the canal, according to two senior U.S. officials who said the United States had been tracking the ship along with the Panamanians for some time.
Another senior U.S. official said the United States had been tracking the ship for several days and knew that Panamanian authorities were going to stop it.
State Department spokesman Patrick Ventrell declined to describe U.S. interactions with Panama concerning the ship, but noted that the vessel has a checkered past connected with drug smuggling.
"Public reports from 2010 and also a U.N. panel of experts report from 2012 cite this history," he said Tuesday. "So this vessel has a well-known history in this regard."
Concerns over Cuba
Cuban state media reported late last month that North Korean army Chief of Staff Gen. Kim Kyok Sik visited the island and had high-level meetings, including one with Cuban leader Raul Castro.
In the United States, Rep. Ileana Ros-Lehtinen, R-Florida, described this week's incident as "serious and alarming" and a "wake-up call" for the Obama administration to avoid normalizing U.S. relations with Cuba.
Some analysts described the situation as a troubling sign that North Korea could be supplying Cuba with weapons.
"This is a country which is just 90 miles away from American shores," Forbes.com columnist Gordon Chang told CNN's "Erin Burnett: OutFront."
"Now, if they can smuggle missile radar into Cuba, you know, God knows what else they can put there. We do not need a replay of the Cuban missile crisis, this time with the North Koreans' fingers on the triggers instead of the Soviets."
Chang, author of "Nuclear Showdown: North Korea Takes On the World," said the Panamanian president's dramatic description of how the ship's crew members handled the incident didn't surprise him.
"They do not want anybody on their ships," he said. "Whether it's carrying melons or nuclear technology, the North Koreans would act pretty much the same way."
A history of weapons shipments
This isn't the first time North Korea has been linked to shipping suspected of transporting weapons materials.
In 2011, the U.S. Navy tried -- and failed -- to gain permission to board a ship in the South China Sea suspected of carrying illicit weapons technology to Myanmar, the Pentagon said. The Belize-flagged MV Light was believed to have been manned by a North Korean crew, the Pentagon said. Under U.S. Navy surveillance, the vessel eventually turned around and headed to North Korea.
In 2007, the Pentagon confirmed that several shipments of suspected weapons technology had left North Korea destined for Syria. The Pentagon said some of the material was believed to have been high-grade metals that could be used to build missiles or solid-fuel rockets.
CNN reported in 2011 that an unpublished U.N. report claimed North Korea was trading banned weapons technology with several countries, including Iran.