Yonhap news agency, citing a top South Korean government official, said two intermediate Musudan missiles had been transported by train earlier in the week and "loaded on vehicles equipped with launch pads".
The Defence Ministry, which on Thursday had confirmed the movement of one missile with "considerable range", declined to comment on the new report.
But a Navy official told Yonhap that two South Korean Aegis destroyers with advance radar systems had been deployed - one off the east coast and one off the west coast to track any missile launch.
"If the North fires off a missile, we will trace its trajectory," the official said.
North Korea, incensed at fresh UN sanctions and South Korea-US military drills, has issued a series of apocalyptic threats of nuclear war in recent weeks.
White House spokesman Jay Carney said Thursday the barrage of rhetoric fitted a "regrettable but familiar" pattern of North Korean behaviour.
"We're taking all the necessary precautions," Carney said, citing "prudent measures" to respond to the possible missile threat.
The Musudan has never been tested, but is believed to have a range of around 3,000 kilometres (1,860 miles), which could theoretically be pushed to 4,000 with a light payload.
That would cover any target in South Korea and Japan, and possibly even reach US military bases located on the Pacific island of Guam.
The senior government official told Yonhap that the mobile launchers had since been hidden in special underground facilities.
"The North is apparently intent on firing the missiles without prior warning," the official said.
The Pentagon has said it will send missile-interceptor batteries to protect its bases on Guam, a US territory some 3,380 kilometres (2,100 miles) southeast of North Korea and home to 6,000 American military personnel.
Most experts think the North is not yet capable of mounting a nuclear device on a ballistic missile which could strike US bases or territory.
On Thursday the North Korean army said it had received final approval for military action, possibly involving nuclear weapons, against the threat posed by US B-52 and B-2 stealth bombers participating in joint military drills with South Korea.
"The moment of explosion is approaching fast," the army's general staff said.
The blistering rhetoric has stoked international concern, with UN chief Ban Ki-moon describing the daily threats from Pyongyang as "really alarming and troubling". (Read: UN warns of 'grave concern')
There has been speculation that Pyongyang might schedule a firing to coincide with the birthday of the country's late founder Kim Il-Sung in mid-April.
"A flight test would make sense," said Daniel Pinkston, a North Korea expert at the International Crisis Group.
"But I'd be surprised if they used an untested missile. At this stage in the game, they don't want to be firing off something that might disintegrate after 30 seconds," Pinkston told AFP.
Tensions have soared on the Korean peninsula since December, when the North test-launched a long-range rocket. In February, it conducted its third nuclear test and drew fresh UN sanctions.
The North also warned this week it would reopen its mothballed Yongbyon reactor - its source of weapons-grade plutonium that was closed in 2007 under an aid-for-disarmament accord.
On Thursday, North Korea blocked access to its Kaesong joint industrial zone with South Korea for the second day running, and threatened to pull out its 53,000 workers in a furious reaction to the South's airing of a "military" contingency plan to protect its own workers there.
The Unification Ministry said there were still 608 South Korean citizens in Kaesong, which was shut Friday for a scheduled North Korean holiday.