North Korea test-fired what appeared to be two intermediate range ballistic missiles on Thursday, but both failed, the US military said.
The isolated nation has conducted a series of missile launches in violation of UN resolutions ahead of the Workers' Party congress which begins on May 6. South Korea fears the North may be ready to conduct a nuclear test at any time.
China said the UN Security Council was working on a response to North Korea's latest missile tests, while UN Secretary-General Ban Ki-moon urged Pyongyang to cease "further provocative actions."
Thursday's tests seemed to have been hurried, according to a defence expert in Seoul, and follow a failed launch of a similar missile earlier this month.
The first launch, at about 6:40 am local time (2140 GMT Wednesday) from near the east coast city of Wonsan, appeared to have been of a Musudan missile with a range of more than 3,000 km (1,800 miles) which crashed within seconds, a South Korean defence ministry official said.
Later, at around 7:26 pm, the North shot a similar intermediate range missile from the same area, but the launch was also understood to have failed, the official added.
The US military's Strategic Command said it tracked two attempted launches, neither of which posed a threat to North America.