South Korea's military is also keeping a close watch on movements by the North's military after detecting signs that Pyongyang appears to be preparing for live-fire drills off its western coast near the tense Yellow Sea border ahead of the planned rocket launch, or possibly this weekend, said another source at Seoul's military.
"North Korea installed the first-stage rocket onto the launch pad," the government source said on the condition of anonymity. "That means North Korea is starting its process of launching a long-range missile."
It will take three or four days for North Korea to erect all three stages of the long-range rocket, South Korean intelligence and military officials say.
Confirming weeks of speculation, North Korea said Saturday it will launch a long-range rocket between Dec. 10 and 22 to put what it calls a "working satellite" into orbit, with much of the world suspecting it is in reality testing inter-continental ballistic missiles capable of carrying nuclear warheads.
The planned launch will be the North's second launch attempt under current leader Kim Jong-un, following a failed launch in April. The young leader took power following his father's death last December.
South Korean satellites and military intelligence assets have confirmed the first-stage of the North's Unha-3 rocket is on the launch pad at Dongchang-ri in the country's far northwest, the source said.
If all goes as planned, including the installation of the other two stages and fuel injection, North Korea will be ready to launch the rocket before Dec. 10, according to the source.
In a move to further ratchet up tensions ahead of the planned rocket launch, a military source in Seoul said North Korea appears to be preparing for naval live-fire drills in waters near the Yellow Sea border.
"After analyzing signs detected from last week, the drills are highly likely to take place around this weekend," the military source said.
About two weeks before the North's failed rocket launch in April this year, Pyongyang test-fired two short-range missiles off its western coast.
North Korea notified its neighbors that the planned launch will take place between 7 a.m. and noon on any day in the given window, Seoul's defense ministry officials said.
According to the notification, the first stage should fall into the Yellow Sea, about 20 kilometers south of the trajectory given ahead of the April launch, and the second should come down in waters about 190 kilometers east of the Philippines. according to the officials.
South Korean and the U.S. militaries are considering upgrading their alert status on North Korea to the next level as the North pushes ahead with its rocket launch plan. About 28,500 American troops are stationed in South Korea.
Defense ministry spokesman Kim Min-seok told reporters, "Our military is closely watching all activities with regard to North Korea's long-range rocket while firmly maintaining military posture and joint alert status with the U.S. military."
Japan has also started deploying a surface-to-air missile defense system ahead of the planned launch, officials said.
The North's announcement it would launch the rocket came a day after young leader Kim met a delegation sent by the new Chinese leader Xi Jinping, with some analysts speculating Pyongyang might ignore Beijing's request to scrap any possible rocket launch.
South Korea is ramping up its diplomatic efforts with the United States, Japan and other neighbors to try to press North Korea to scrap its plan to launch the rocket this month, Seoul's foreign ministry officials said.
First Vice Foreign Minister Ahn Ho-young met with U.S. Ambassador to Seoul Sung Kim on Monday and Lim Sung-nam, Seoul's chief nuclear envoy, held separate talks with ambassadors from China, Russia and Japan the same day, foreign ministry officials said.
Chinese Ambassador Zhang Xinsen, after talks with Lim, told reporters the two had held "in-depth and wide-ranging discussions on issues of mutual concern."
Lim also plans to travel to the U.S. Tuesday to discuss with his U.S. counterpart, Glyn Davies, how to get North Korea to drop its launch plan, officials said.
"Our primary aim is to get North Korea to give up its plan to launch a long-range missile," a senior ministry official said on the condition of anonymity.
Foreign ministry spokesman Cho Tai-young said Lim, during his three-day visit, "will consult with U.S. officials about the current situation on the Korean Peninsula as well as North Korea and its nuclear issues, including ways to respond to North Korea's plan to launch a long-range missile announced on Dec. 1."
The meetings with officials from the U.S., China, Russia and Japan were also aimed at studying countermeasures to take if North Korea goes ahead with its threatened rocket launch, the official said.
There is deep uncertainty, however, whether such diplomatic efforts can convince the North to scrap its rocket launch plan, analysts say.
South Korea expressed "grave concern" over the North's rocket launch plan "in the name of a working satellite," calling it a "grave provocation."
North Korea "will be confronted with a strong response from the international community as clearly stated in the U.N. Security Council Presidential Statement in April," which was adopted after the North's failed rocket launch, the foreign ministry spokesman Cho said in a statement.
U.S. State Department spokeswoman Victoria Nuland called the North's rocket launch plan a "highly provocative act that threatens peace and security in the region," and warned, "Devoting scarce resources to the development of nuclear weapons and long-range missiles will only further isolate and impoverish North Korea."
China, the North's only major ally, expressed "concern" about the North's rocket launch plan.
The North's decision to launch another long-range rocket this month coincides with many occasions in the region.
South Koreans go to the polls on Dec. 19 to pick their new leader and Japan is also set to hold general elections within two weeks. North Korea will also mark the first anniversary of the death of Kim Jong-il, the late father of current leader Kim Jong-un.
The North's rocket launch plan also comes weeks after U.S. President Barack Obama won his second term and China's president-in-waiting Xi Jinping secured top posts of the Chinese Community Party and its military in a once-in-a-decade power transfer.