Japan and South Korea said the test was not a major worry since it appeared the weapon was a short-range missile incapable of carrying a nuclear warhead.
Officials, however, said on Monday that the test would strain efforts to restart talks on Pyongyang's nuclear programme.
North Korea test-launched what appeared to be a short-range missile into the Sea of Japan on Sunday, US officials had earlier said. The test came as Washington indicated that Pyongyang might be headed towards a nuclear test.
"Is this the kind of missile that can carry a nuclear warhead? Not really," a South Korean government official said on condition of anonymity.
A Defence Ministry spokeswoman in Tokyo said Japan had yet to confirm whether the launch took place.
"At this point, the missile in question is thought to have flown a very short distance and cannot be described as something that immediately has a particular impact on our country's security," she said.
Tokyo believes the weapon may have been a land-to-ship or small ballistic missile. The missile, which probably had a range of about 100km, may have been launched from North Korea's east coast, Japanese network NHK reported, quoting unnamed defence sources.
Pyngyang's neighbours say the
test is not a major worry
Analysts said Pyongyang might have meant to send a jolt to a six-party process aimed at ending its nuclear ambitions. The talks have stalled since June 2004.
North Korea is currently free of international surveillance of its nuclear activities. It kicked out International Atomic Energy Agency (IAEA) inspectors in December 2002, withdrew from the nuclear Non-Proliferation Treaty (NPT) the following month and now claims to have made atomic bombs.
No threat increase
"This missile launch does not mean that there has been an increase in the threat from North Korea," said Noriyuki Suzuki, chief analyst at Tokyo-based Radiopress, which monitors North Korea.
Suzuki noted when North Korea fired a short-range missile last year, it was trying to increase its leverage in the talks and added Pyongyang might have had a similar intention this time.
US believes the North may be
trying to build nuclear missiles
"The overt purpose is an exercise or test, but given the timing, there could be an element of provocation, a political element," he said.
The missile test caused the Japanese yen and the South Korean won to fall in early trading on Monday, but had little impact on stocks.
Earlier, the White House called the test a bullying tactic and said there was growing evidence that Pyongyang might be working to arm missiles with nuclear warheads.
The suspected North Korean missile test came on the eve of a UN-hosted conference expected to focus on nuclear proliferation concerns, including North Korea.
Review of NPT
Meanwhile, about 190 nations have begun a review of the NPT, with worsening crises in North Korea and Iran showing how difficult the world's fight against the spread of atomic weapons is.
The treaty, which went into effect in 1970, seems flawed, if not outright ineffective, ahead of the conference at the United Nations.
The world is facing a new era of
nuclear 'rogue' states
Since the treaty was signed, the world faces a new era of "rogue" states, international nuclear smuggling rings, and trans-national terrorist groups seeking weapons of mass destruction.
Iran is showing the strains in the non-proliferation treaty in another way as the United States claims the Islamic Republic is secretly developing atomic weapons under the cover of a civilian nuclear power programme that is under IAEA safeguards.
Iran's supreme leader, Ayatollah Ali Khamenei, on Sunday dismissed Washington's concerns over Tehran's nuclear programme, the day after Iran said it was unhappy with the progress of nuclear negotiations with Britain, France and Germany, and warned it might resume uranium conversion activities in defiance of a November agreement.
The European Union, backed by the US, wants Iran to halt all nuclear fuel cycle activities. In return, the EU is offering a package of trade, security and technology incentives.