US Secretary of State Condoleezza Rice is in South Korea on the second leg of an Asian tour to rally opposition to North Korea's nuclear testing.
Ms Rice told Mr Abe the US would honour defence commitments
In an earlier meeting with Japan's prime minister, she again warned the North not to carry out a second test.
Her trip follows a UN Security Council vote backing sanctions in response to North Korea's 9 October test.
Correspondents say Ms Rice will face a more wary reception in the South, which favours engagement with its neighbour.
A North Korean official gave the country's first indication it may be preparing a second nuclear test.
The deputy head of North Korea's foreign ministry, Li Gun, speaking on ABC TV in the US, said a second test would be "natural" and that the US should not be surprised if one were carried out.
President George W Bush warned North Korea it would face "grave consequences" if it tried to transfer nuclear weapons.
Ms Rice first met Japanese Prime Minister Shinzo Abe in Tokyo.
N KOREA NUCLEAR PROGRAMME
Believed to have 'handful' of nuclear weapons
But not thought to have any small enough to put in a missile
Could try dropping from plane, though world watching closely
Amid concern in Washington that the crisis could encourage other Asian nations to develop their own nuclear weapons programmes, Ms Rice stressed that the US was ready to act on its defence commitments to Japan.
Japan's Foreign Minister Taro Aso said Tokyo was "absolutely not considering a need to be armed by nuclear weapons".
Japan's Kyodo news agency said the US and Japan had agreed to examine the role of the military in searching North Korean cargo ships.
Ms Rice and Mr Aso later flew on to Seoul to rally support for the full implementation of sanctions in line with the UN resolution.
But the BBC's Charles Scanlon in Seoul says Ms Rice can expect a wary reception from the South, which is pushing for a less confrontational approach and still favours a policy of reconciliation and economic engagement.
NEW UN SANCTIONS
Bans sale to, or export from, N Korea of military hardware
Bans sale or export of nuclear and missile related items
Bans sale of luxury goods
Freezes finances and bans travel of anyone involved in nuclear, missile programmes
Allows inspection of cargo to and from N Korea
Stresses new resolution needed for further action
However, the South Korean Yonhap news agency said Seoul was preparing to bolster the inspection of cargo heading to the North and to block subsidies to a joint tourism project there.
A US diplomat said Ms Rice would again press the South on the US-led Proliferation Security Initiative which was set up in 2003 to inspect ships suspected of carrying materials that could be used for weapons of mass destruction.
The South has been reluctant to join for fear of sparking conflict with the North.
Ms Rice's tour also takes her China, which is thought to have even greater concerns over inspecting North Korean shipping in case it triggers a confrontation.
The tour comes amid concerns that the North might conduct another nuclear test.
A member of South Korea's parliamentary intelligence committee, Chung Hyung-keun, said the North could be preparing three or four more tests.
Ms Rice in Tokyo warned of "further measures" if the North tested again.
US officials said a high-ranking Chinese delegation, led by former Foreign Minister Tang Jiaxuan, was in North Korea and was expected to press the North not to test again.
In a television interview, President Bush said the US would use whatever means necessary to prevent the North transferring nuclear weapons to third parties such as Iran or al-Qaeda.
"We would stop the transfer, and we would deal with the ships... or the airplane that was... taking the material," Mr Bush told ABC News.