North Korea's latest ballistic missile launches will be condemned by the UN Security Council, US Ambassador Samantha Power predicted following an emergency meeting.
She told reporters after the closed meeting there were "strong condemnations across the board" from all 15 council members, which she found encouraging.
However UN diplomats said attempts to condemn the launches have been held up by China.
South Korean and Japanese officials said a medium-range ballistic missile fired on Wednesday flew about 620 miles and landed near Japan's territorial waters.
It was one of the longest flights by a North Korean missile, and the first time a missile landed in Japan's exclusive economic zone, which extends 200 nautical miles from shore.
The US Strategic Command said initial indications were that a second missile exploded immediately after launch.
Britain's deputy UN ambassador Peter Wilson said as he headed into the emergency council meeting they were concerned that it was the first time a North Korean missile had landed in Japan's EEZ.
"Our objective in this meeting is to draw attention to that fact, but also to come together to very, very clearly condemn it," he said.
After the meeting, Japan's UN Ambassador Koro Bessho was heartened by the "many messages of solidarity" after his appeal to the council to be united and send a strong message to the world and especially to North Korea that its actions are "totally unacceptable".
No date was announced for a follow-up meeting of council members or experts, but Mr Bessho said he hopes for council action as soon as possible.
South Korea's UN Ambassador Oh Joon said so far this year North Korea has conducted 13 rounds of missile tests - including the firing of 29 missiles of different varieties, ranges and trajectories using mobile launchers and submarines.
"They are doing all of this with a systematic, comprehensive purpose of upgrading and refining their missile technologies, which is not only a grave challenge to the global nonproliferation system but also poses a clear and present danger to the security of all countries in the region," he said.
"It is definitely in the interest of all countries to stop this dangerous series of provocations immediately."
The ballistic missile launches on Wednesday were the third since the US and South Korea announced plans on July 8 to deploy an advanced missile defence system in South Korea by the end of next year - a move Pyongyang has denounced.
The council has condemned six other launches in April, May and June, but has not responded to a July 9 failed attempt by a North Korean submarine to launch a ballistic missile and the July 19 launch of two short-range Scud missiles and a Rodong intermediate-range missile.
China's UN Ambassador Liu Jieyi told reporters that "the situation is tense and we need to do everything to de-escalate the situation".
Ms Power said the South Korea missile defence system was "purely defensive" and aimed solely at protecting the South Korean people and US and other forces stationed there from North Korean ballistic missiles.
She stressed that the US will be pushing for implementation and enforcement of the sanctions - and is looking at adding individuals and organisations to the sanctions blacklist, which requires approval by all 15 council members.