The UN Security Council has scheduled an emergency meeting on North Korea's latest missile tests at the request of the United States and Japan.
South Korean and Japanese officials said one ballistic missile fired on Wednesday landed near Japan's territorial waters.
US officials said initial indications were that a second missile exploded immediately after launch.
Britain's deputy UN ambassador Peter Wilson said this was the first time a North Korean missile had landed in Japan's exclusive economic zone.
"I think this is something the council should take extremely seriously," Mr Wilson said. "This is a clear violation of Security Council resolutions."
UN spokesman Stephane Dujarric said the tests "seriously undermine regional peace and stability".