BEIJING — North Korea's new young leader will have to share power with an uncle and the military after the death of his father Kim Jong Il as the isolated country shifts to collective rule from strongman dictatorship, a source with close ties to Pyongyang and Beijing told Reuters.

The source added that the military, which is trying to develop a nuclear arsenal, has pledged allegiance to the untested Kim Jong Un, who takes over the family dynasty that has ruled North Korea since it was founded after World War Two.

The source also said Beijing was only notified of Kim's death earlier on Monday, the same day that North Korean state television broadcast the news. Kim died on Saturday from a heart attack, aged 69.

Reuters said the source declined to be identified but had correctly predicted events in the past, including the North's first nuclear test in 2006.

Pledge of allegiance
The situation in North Korea appeared stable after the military gave its backing to Kim's son and successor, Kim Jong Un, the Reuters source said.

"It's very unlikely," the source said when asked about the possibility of a military coup. "The military has pledged allegiance to Kim Jong Un."

Tens of thousands of mourners packed Pyongyang's snowy main square Wednesday to pay respects to late leader.

Women held handkerchiefs to their faces as they wept and filed past a huge portrait of a smiling Kim Jong Il hanging on the Grand People's Study House, in the spot where a photograph of Kim's father, North Korean founder Kim Il Sung, usually hangs.

Kim Jong Il died of a heart attack Saturday, according to state media, which reported his death on Monday.

With no military strongman, North Korea will be ruled by collective leadership, including Kim Jong Un, his uncle and the military, the source said.

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Jang Song Thaek, 65, brother-in-law of Kim Jong Il and the younger Kim's uncle, was named in 2009 to the National Defense Commission, the supreme leadership council Kim Jong-il led as head of the military state.

The source also said the North Korea test-fired a missile on Monday to warn the United States not to make any moves against it. Pyongyang also had no immediate plans for further tests barring an escalation of tensions.

"With the missile test, (North) Korea wanted to deliver the message that they have the ability to protect themselves," the source told Reuters.

"But (North) Korea is unlikely to conduct a nuclear test in the near future unless provoked" by the United States and South Korea, the source said.

N. Korea troops on alert
South Korean intelligence sources told the Associated Press that North Korea had tightened security in cities and won loyalty pledges from top generals.

Seoul's National Intelligence Service believes the North is now focused on consolidating Kim Jong Un's power and has placed its troops on alert since Kim Jong Il's death, according to South Korean parliament member Kwon Young-se.

In a move likely to infuriate the North, South Korean activists and defectors launched giant balloons containing tens of thousands of propaganda leaflets on Wednesday, some of them opposing a hereditary transfer of power in North Korea.

Kim Jong Il ruled the country for 17 years after inheriting power from his father, national founder and eternal North Korean President Kim Il Sung, who died in 1994. Kim Jong Un only entered the public view last year and remains a mystery to most of the world.

Slideshow: The life of Kim Jong ll (on this page)

The North's nuclear program has been a nagging source of tensions with the international community. Pyongyang carried out nuclear tests in 2006 and 2009, and has quit six-party talks with South Korea, the United States, China, Japan and Russia on abandoning its nuclear program and returning to the Non-Proliferation Treaty.

China, the North's closest ally and biggest provider of aid, on Tuesday invited the new North Korean leader to visit after his father's death. Chinese President Hu Jintao and Vice-President Xi Jinping also visited the hermit state's embassy in Beijing to express their condolences on Tuesday.

The fear of instability on its northeastern border worries China. North Korea has been pressed by China to denuclearize and is willing to do so on condition that North and South Korea, the United States and China sign a deal replacing a 1953 truce, the source said.

The two Koreas have been divided for decades and remain technically at war since their 1950 to 1953 conflict ended with an armistice but no peace agreement. The United States backed the South, while China supported the North in that conflict.

Pyongyang also is convinced there are U.S. nuclear weapons in South Korea and demands Washington pull them out, the Reuters source said.

Dr. John Swenson-Wright, associate fellow of the Asia program at London-based think tank Chatham House, said any hope of a deal to formally end the war between the two countries was "premature."

"That would require a much more comprehensive settlement, involving (from Pyongyang's point of view) the full withdrawal of U.S. forces from the peninsula, and probably also the region as a a whole," he told msnbc.com.

"The requirement that the U.S. effectively end its security alliances with [South Korea] and Japan is clearly a nonstarter, so the idea of a treaty opening up the door to full denuclearization is clearly not going to work," Swenson-Wright added.

"At the moment, given the insecurities and uncertainties in the North regarding who is in power, I think talk of a peace treaty is premature," he added.

Reuters and the Associated Press contributed to this report.