South Korea has raised its alert level to “vital threat” amid indications the North is preparing for a missile test.
At least one ballistic missile with an estimated 3,000km (2,000-mile) range is fuelled and ready for launch, US and South Korean sources say.
Pyongyang has been making bellicose threats against South Korea, Japan and US bases in the region.
The threats follow tough new UN sanctions imposed on North Korea last month after its third nuclear test.
Separately, an initial investigation by the South into a major cyber attack last month that affected a number of banks and broadcasters has said the North is to blame.
North Korea is believed to have completed preparations for a missile launch after it moved two Musudan missiles to its east coast, Yonhap news agency says.
In anticipation, the South Korea-US Combined Forces have raised their alert level to Watchcon 2, to increase surveillance monitoring, Yonhap quoted a senior military official as saying.
North Korea unveiled the Musudan missile during a military parade in 2010 but has yet to test it. There are reports, however, that it may have been sold to Iran and tested there.
The launch could happen “anytime from now”, South Korean Foreign Minister Yun Byung-se told parliament.
A test launch would be a violation of UN Security Council Resolution 1718, passed in 2006, which states the North “must not conduct any further nuclear test or launch of a ballistic missile”.
North Korea has tested intermediate range missiles before and during periods of crisis and tension, says the BBC’s John Sudworth in Seoul.
So while another test launch would certainly be seen as provocative, it is unlikely to have any major, short-term military significance unless it goes wrong, our correspondent adds.
He says one key date for a launch could be Monday – the birthday of the North Korean state’s late founder, Kim Il-sung.
Yonhap also reported that the South’s National Police Agency had raised its state of alert by one level from “attention” to “caution”.
It said that patrols had been increased at 770 sites, including at embassies and key underground stations.
Japan’s Defence Minister Itsunori Onodera said his nation was on “high alert”, with anti-missile defences deployed in Tokyo as a precaution.
A number of travel agencies in China have reported that tourist trips into North Korea have been suspended.
One travel agent in the north-eastern city of Dandong told Reuters news agency: “All [tourist] travel to North Korea has been stopped from today and I’ve no idea when it will restart.”
The border remains open to commercial traffic.
Meanwhile, an official investigation by the South into last month’s cyber attack traced the malicious codes used to six computers in the North.
“We’ve collected a lot of evidence to determine the North’s Reconnaissance General Bureau led the attack, which had been prepared for at least eight months,” a spokesman for the Korea Internet and Security Agency said.
The attack on 20 March severely affected the KBS, MBC and YTN broadcasters and operations at the Shinhan, NongHyup and Jeju banks.
The North has warned foreigners in South Korea to take precautions in case of war.
On Tuesday, UN Secretary-General Ban Ki-moon warned the crisis on the Korean peninsula may become “uncontrollable”.
He once again urged North Korea to tone down its “provocative rhetoric” and to keep open a joint North-South Korean industrial complex.
North Koreans failed to report for work at the Kaesong complex on Tuesday, suspending one of the few points of co-operation with South Korea.
Since the UN sanctions were imposed, Pyongyang has threatened to use nuclear weapons and has said it will restart a nuclear reactor.
The North has also shut an emergency military hotline between Seoul and Pyongyang.
Last week, it warned it would not be able to guarantee the safety of foreign embassy staff after 10 April, and that countries should begin evacuating their diplomatic staff.