US President Donald Trump announced the “heaviest” set of US sanctions against North Korea on Friday as his administration redoubled its efforts to starve Pyongyang of resources and force the isolated regime to abandon its nuclear ambitions.

“We imposed today the heaviest sanctions ever imposed on a country before,” Trump said at the close of a wide-ranging address to a conservative political conference, The Washington Post reported.

The new measures target 56 vessels, shipping companies and other entities that Trump administration officials believe are used by North Korea to conduct trade prohibited under previous sanctions.

Significant advances in its missile and nuclear weapons programs have made North Korea the most pressing foreign threat facing the Trump administration.

The punitive measures, which follow multiple rounds of earlier sanctions, signal US officials’ determination to use financial pressure to prompt North Korean leader Kim Jong-Un to relinquish nuclear weapons, and their concern that under-the-radar fossil fuel trade undermines their ability to do so.

Trump made the announcement in the final moments of a more than hour-long address at the Conservative Political Action Conference outside Washington. “I love you, I respect you, I appreciate everything you have done for the country,” the President said, pausing as the crowd applauded and a few chanted “U-S-A! U-S-A!”

“I do want to say, because people have asked: North Korea,” he said before briefly mentioning the sanctions. “Hopefully, something positive can happen. We will see.”

Senior US officials described elaborate steps that North Korea has taken to conceal its illicit shipping activities, including conducting prohibited ship-to-ship transfers, falsifying ship names and disabling vessels’ automatic identification systems “to try to intentionally mask their movements.”

“We are determined through these efforts to increase the pressure and show Kim Jong Un that there is no other path to take but denuclearization,” one official, who like others spoke on the condition of anonymity, told reporters ahead of Trump’s remarks.