U.S. efforts to sustain and possibly intensify its "maximum pressure" campaign on North Korea are entering a perilous phase as a potential breakdown in diplomacy with Pyongyang raises fears that China may loosen its enforcement of international sanctions.

With the fate of a summit between President Donald Trump and North Korean leader Kim Jong Un in limbo, U.S. officials have suggested Washington may pursue fresh sanctions on Pyongyang over its nuclear program even though key players such as China and South Korea may be reluctant to participate.

Another way for the United States and its allies to increase pressure on North Korea could be to step up efforts to intercept ships suspected of violating international trade restrictions on Pyongyang. One U.S. official, speaking on condition of anonymity, said more countries are interested in joining that initiative.