US President Donald Trump met with North Korea's Kim Jong Un on Sunday. (File)

North Korea complained on Wednesday after the United States sent a letter urging countries to send back North Korean workers as US President Donald Trump was inviting Kim Jong Un to hold talks.

The North Korean mission to the United Nations said the letter sent to all UN member-states showed that the United States is "practically more and more hell-bent on the hostile acts" against North Korea, even though it is seeking dialogue.

The mission said the letter from the United States, along with Britain, France and Germany, was sent on June 29, the day Trump tweeted that he would like to shake Kim's hand and say hello during his visit to the demilitarized zone on the Korean peninsula.

Trump met with Kim on Sunday, becoming the first US president to step onto North Korean soil at the demilitarized zone. He said the two leaders agreed to start working-level talks on a denuclearization deal, ending a standstill.

"What can't be overlooked is the fact that this joint letter game was carried out by the permanent mission of the United States to the UN under the instruction of the State Department, on the very same day when President Trump proposed (for) the summit meeting," said a press statement from the North Korean mission.

The US letter was in fact sent on June 27 and called on all countries to apply sanctions provisions that call for the return of all North Korean workers by the end of 2019.

UN experts estimate that tens of thousands of North Koreans are sent abroad every year, mostly to China and Russia, working in slave-like conditions to generate hard currency revenue for Pyongyang.

"We do not thirst for lifting of sanctions," the North Korean mission said, adding that it was "quite ridiculous" for the United States to consider sanctions as a "panacea for all problems."

UN sanctions resolutions have banned all new contracts with North Korean workers and states all countries with existing workers will send them back by the end of 2019.

The UN Security Council has adopted a series of sanctions in response to the North's ballistic missile and nuclear tests.

In the letter, the four countries noted that only 34 countries had filed reports to the United Nations on whether action was taken to send North Korean workers back.

The final deadline for repatriating North Korean workers is December 22, according to the letter seen by AFP.