North Korea has installed the first stage of its three-stage, long-range rocket on its launch platform, a Seoul government source said Monday, pushing ahead with its plan to fire the rocket this month in defiance of international warnings.

"North Korea installed the first-stage rocket onto the launch pad," the 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.

South Korea's military is considering upgrading its alert status on North Korea to the next level as the North pushes ahead with its rocket launch plan, the source said.

Japan has also started deploying a surface-to-air missile defense system ahead of the planned launch.

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 is due to meet with U.S. Ambassador to Seoul Sung Kim on Monday and Lim Sung-nam, Seoul's chief nuclear envoy, will meet with ambassadors from China, Russia and Japan on the same day, foreign ministry officials said.

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.

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, foreign ministry spokesman Cho Tai-young 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. (Yonhap News)