The North has previously test-fired short-range missiles into the Yellow Sea and the Sea of Japan, often during periods of tension in the region.
North Korea conducted its second nuclear test on Monday and followed it up with a series of short-range missile launches, all in violation of a UN resolution.
But despite the growing tensions, the US defence secretary has said that Washington is not planning to add to the 28,000 US troops already stationed on the Korean peninsula.
Robert Gates said that the situation had not yet reached crisis levels and there had been no unusual moves by the North Korean military since its carried out the nuclear test.
"I don't think there is a need for us to reinforce our military presence in the South. Should the North Koreans do something extremely provocative militarily, then we have the forces to deal with it," he said.
Even so, the US-South Korea Combined Forces Command raised its alert level from stage three to two on Thursday.
The alert level is the highest since North Korea's first nuclear test in 2006.
General George Casey, the US army's most senior officer, said on Thursday that the US could fight a conventional war against North Korea's million-strong military and could be deployed rapidly if necessary, despite already being engaged in wars in Afghanistan and Iraq.
The UN Security Council is still to respond to the tests, but on Thursday it circulated a draft resolution condemning North Korea and urging member states to enforce previously approved sanctions against Pyongyang.
Envoys from Japan and South Korea met their counterparts from the five permanent members - Britain, China, France, Russia and the US - in closed-door talks on Thursday to discuss the draft that would condemn the test "in the strongest terms".
But diplomats revealed little detail when they emerged, saying they needed more time.
Vitaly Churkin, Russia's ambassador, said there was wide agreement on what a new resolution should include, but cautioned that getting agreement would take time.
John Sawers, the British ambassador, echoed that view, telling reporters after the meeting that "we need some time" as "this is quite a complicated discussion".
There was no word on whether China, seen as North Korea's ally, would back the resolution.
Yukio Takasu, the ambassador for Japan which is drafting the text together with the US, would only say that "China understands the seriousness of the situation".