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 United States has 28,000 troops stationed in South Korea.
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.
There were also reports that it had restarted its Yongbyon nuclear plant and the North threatened "merciless punishment" against any attempt by US-led forces to intercept its vessels off the Korean peninsula's west coast.
Pyongyang on Friday vowed to respond if the UN Security Council imposes fresh sanctions following the tests.
"If the UN Security Council provokes us, our additional self-defence measures will be inevitable," North Korea's foreign ministry said in a statement.
"Any hostile acts by the UN Security Council (UNSC) will be tantamount to the demolition of the armistice," it said in a reference to the truce that ended the Korean War in 1953.
The UN Security Council is still to respond with any punitive action, but appeared on Thursday to be moving closer to passing a resolution to condemn the nuclear test.
The council circulated a draft resolution that condemns the test and urges member states to enforce previously approved sanctions against Pyongyang, but stops short of calling for new sanctions.
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".