"President Moon said he understands why the United States has traditionally said all options were on the table to push North Korea to give up its nuclear ambitions".
Leader Kim Jong-Un last week delayed the Guam strike plan, but warned it could go ahead depending on Washington's next move.
Ulchi-Freedom Guardian is referred to a combined military exercise between the United States and South Korea, which is considered the world's largest computerized command and control implementation.
While the allies are pushing ahead with the exercises that date back to 1976, around 17,500 USA troops will participate in the drills - a cutback from past year.
"I worry it might enact this plan when the joint exercises start", said James Acton, co-director of the Nuclear Policy programme at the Carnegie Endowment for International Peace.
China urges "relevant parties" to "give serious consideration" to its proposal of North Korea suspending its nuclear programme in exchange for the USA and South Korea halting their annual military drills, Hua said.
Seoul and Washington began annual war games on Monday, with South Korean President Moon Jae-In warning the nuclear-armed North not to use them as an excuse to perpetuate the "vicious cycle" of tensions.
Vipin Narang, an associate professor at the Massachusetts Institute of Technology specialising in nuclear strategy, warned against another suspension.
The admiral arrived in South Korea earlier in the day to discuss North Korean threats and inspect the allies' joint military drills.
It also said, "Australia followed the USA to the Korean War, the Vietnamese War and the 'war on terrorism, ' but heavy loss of lives and assets were all that it got in return". He said the military exercises are "not now on the table as part of the negotiations in any way".
Joint exercises happen twice a year - once in March, again in August.
But it declined to comment on whether the drills would be scaled back in an effort to ease tensions.
Pyongyang conducted two tests of its long-range missile in July and had threatened to fire four ballistic missiles toward Guam.
The North Korean leader "said that the US imperialists put their own necks into the noose through their reckless military confrontation racket, adding that he would watch a little longer the conduct of the foolish and stupid Yankees", the ambassador told Guterres.
Any negotiation for a quiet agreement to reduce the frequency and range of the B-1B flights "doesn't have to be terribly public and it could be behind the scene", he said.
North Korea appears to have left open some room for a quid pro quo, added Narang.