A fleet of North Korean freighters this week returned home with the coal they were supposed to have delivered to Chinese buyers, as Beijing attempts to use economic persuasion to get Pyongyang back in line.

China cut off coal imports from North Korea in February because of its missile tests, cutting off the cash flow from Pyongyang's top export.  But the launches have continued, and there has been increased activity at the Punggye-ri Nuclear Test Site - the type of activity that in the past has preceded an atomic bomb blast.

A total of two million tons of North Korean coal had been trapped at Chinese ports, only to make the return journey back across the Yellow Sea. Compounding China's coal shortfall was tropical Cyclone Debbie, which knocked out supplies from the world's top coking coal region in Queensland.  China is making up the shortfall by purchasing more coal from the United States.  No US coking coal was exported to China between late 2014 and 2016, but shipments soared to over 400,000 tonnes by late February.  Russia is also selling more coking coal to China to fill the void.

A US Navy aircraft carrier strike group is steaming towards the waters off the Korean peninsula.  China and South Korea reached an agreement to slap tougher sanctions on North Korea if it carries out nuclear or long-range missile tests.