Reuters news agency reports the United States has blocked efforts by a UN agency to improve civil aviation in North Korea at a time when Pyongyang is trying to reopen part of its airspace to foreign flights.

According to Reuters, one of three sources familiar with the matter speaking on the condition of anonymity said the U.S. move is part of a negotiating tactic to maintain sanctions pressure on North Korea ahead of a second summit between President Donald Trump and North Korean leader Kim Jong-un later this month in Vietnam.

The report said the UN International Civil Aviation Organization(ICAO) has been working with Pyongyang to open a new air route that would pass through North and South Korean airspace.

Airlines currently take indirect routes to avoid North Korea due to the threat of unannounced missile launches. 

Since Pyongyang's sinking of a South Korean Navy vessel in 2010, Seoul also banned national carriers from passing through North Korean air space. 

Reuters said if the space was deemed safe, international airlines could save fuel and time on some routes between Asia and Europe and North America while North Korea could begin reviving its own commercial aviation industry.

According to sources, the Montreal-based UN agency was prepared to help improve North Korea’s aviation system by leading training sessions between its military and civil aviation staff. The North also reportedly asked ICAO for access to U.S.-produced aeronautical charts.

The United States, however, discouraged the agency from helping North Korea with its air program as it wanted to “pool all the leverages and incentives” until substantial progress is made on denuclearization.

Another source told Reuters the U.S. would keep tight hold of all available leverage to make sure there is no loophole until the North takes action that deserves a reward.