North Korea has test-fired two powerful new Musudan mid-range ballistic missiles, US and South Korean military officials have said.
The launches mark the North's fifth and sixth such attempts since April.
Five of those launches failed, many exploding in mid-air or crashing, while the sixth flew only about 250 miles, South Korean military chiefs said, well short of the missile's potential 2,180-mile range.
The North's determination in testing the Musudan has worried diplomats in Washington, Tokyo and Seoul, because the missile's range puts much of Asia and the Pacific, including US military bases, within reach.
Each new test - apparently linked to a command from North Korean leader Kim Jong Un - also provides valuable insights to the North's scientists and military officials as it pushes towards its goal of a nuclear and missile system which can threaten the US mainland.
Pyongyang conducted a nuclear test, its fourth, this year, and launched a long-range rocket that outsiders say provided cover for a test of banned missile technology.
In April, North Korea made an unsuccessful attempt to launch three suspected Musudan missiles, but all exploded in mid-air or crashed, according to South Korean defence officials.
Earlier this month, North Korea had another suspected Musudan failure.
Before April's launches, North Korea had never flight-tested a Musudan missile, although one was displayed during a military parade in 2010 in the capital, Pyongyang.
The launches appear to stem from Kim Jong Un's order in March for more nuclear and ballistic missile tests. The order was an apparent response to springtime US-South Korean military drills, which North Korea views as an invasion rehearsal.
Since the end of those military drills, Pyongyang has repeatedly called for the resumption of talks with Seoul, even as it pursues new missile development, but the South has rejected the overtures.
Seoul wants the North to first take steps toward nuclear disarmament. Pyongyang says its rivals must negotiate with it as an established nuclear power, something Washington and Seoul refuse to do.
Meanwhile, diplomats from the US and North Korea are attending a six-nation security forum in Beijing in a rare opportunity for contact between the sides.
Despite their joint attendance at Wednesday's event, the US State Department said there are no plans for direct talks between the US special representative for North Korea policy Sung Kim, and North Korea's deputy director general of its Foreign Ministry's department of US affairs, Choe Son Hui.
The conference is described as a multilateral forum involving high-level policymakers, defence ministry officials, military officers and researchers from China, Japan, North and South Korea, Russia, and the United States.
The nations taking part had participated in years of talks on North Korean nuclear disarmament which have been stalled since 2008, with little apparent prospect of a resumption.