Caskets that the USA shipped to the border last month haven't been filled, despite Kim Jong Un's pledge during his June 12 summit with President Donald Trump to immediately repatriate identified remains.
Mr. Pompeo denied that the talks went poorly, telling pool reporters that progress was made "in every element of our discussions".
"I deeply appreciate the energetic and extraordinary efforts made by Your Excellency Mr. President for the improvement of relations between the two countries", the letter read, in part.
While in Brussels for the annual NATO summit on Thursday, both Trump and Pompeo expressed confidence in the diplomatic process with North Korea despite lacking of tangible signs of progress. The US did not say which countries were illicitly providing Pyongyang with the products.
Returning the remains was a commitment from the 12 June summit between Kim Jong-un and Donald Trump in Singapore.
North Korean officials did not attend a planned meeting with Americans to discuss the return of U.S. soldiers' remains, officials have said.
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Kim used the abbreviation for North Korea's official name, the Democratic People's Republic of Korea.
He also agreed to repatriate remains of U.S. troops who died during the Korean War six decades ago. The letter was dated July 6, 2018, and was addressed to "H.E. Donald J. Trump", in what appears to be the latest sign that the two men were getting along after making history at the first-ever meeting between a sitting US president and North Korean supreme leader in Singapore last month.
State Department spokeswoman Heather Nauert said the talks will take place Sunday and the U.S.
But North Korea's delegation failed show up to a scheduled meeting on Thursdayat the inter-Korean truce village of Panmunjom in the Demilitarized Zone between the two Koreas.
Trump has sought to cast his summit with Kim as a major victory, although reports have suggested North Korea is continuing with its nuclear programme and promises of "denuclearisation" remain vague.
The letter contrasts with a statement put out on July 7 by an unnamed North Korean official saying that Washington's attitude and stance were "regrettable" and that the United States "came up only with its unilateral and gangster-like demand for denuclearization".
CNN reported last month that the Trump administration is expecting North Korea to return up to 200 sets of remains and that planning for the exchange has been underway.