North Korea marked the 65th birthday of leader Kim Jong Il on Friday amid progress in ending its nuclear programs and lingering speculation abroad over who will eventually succeed him.

Kim's birthday is one of North Korea's most important national holidays and one in which the personality cult inherited from his father, the country's founder Kim Il Sung, is arguably the most visible.

North Koreans usually receive benefits such as extra food, but it remains unclear whether the country can dole out such largesse this year, given chronic food shortages and U.N. sanctions imposed over its Oct. 9 nuclear test.

"Holidays in North Korea mark occasions on which the leadership is obligated to show tangibly its ability to care for the people," said Scott Snyder, a senior associate at the Asia Foundation in Washington and who formally served as chief of its Seoul office.

The North Korean "leadership will be able to perform at a higher level in this area" amid reduced tensions with the international community following this week's nuclear agreement, Snyder added.

In a breakthrough deal reached in Beijing on Tuesday, the hardline communist regime agreed to shut down its main nuclear reactor and allow U.N. inspectors back into the country within 60 days.

In return, the energy-starved country would receive aid equal to 50,000 tons of heavy fuel oil from the other countries participating in the six-party talks — the United States, South Korea, Russia, China and Japan.

"Psychologically, Kim Jong Il would not be in a somber mood," said Koh Yu-hwan, a North Korea expert at Seoul's Dongguk University, citing the nuclear deal and the prospect of winning economic aid.

Still, North Korea kept up its anti-American rhetoric and urged its people to rally around Kim, known as the country's "dear leader."

In a joint letter of congratulations to Kim on his birthday, North Korea's Cabinet, ruling party, parliament and military vowed to defend the country from the United States.

"All of the People's Army soldiers and the people will maintain a full combat mobilization posture in response to U.S. imperialists' maneuvers for aggression and mercilessly destroy and mop up the aggressors if they dare to ignite a war," the Korean Central News Agency, in a Korean-language report, quoted the letter as saying.

Experts, however, dismissed such harsh language as being aimed at bolstering support for the leadership at home. North Korea regularly says the U.S. is plotting an attack, a charge Washington consistently denies.

"It is a declaration of its will to safeguard its internal system," Koh said of the anti-U.S. rhetoric.

Another version of the letter's contents, carried on KCNA's English-language service, used weaker language and did not mention the United States.

In the run-up to the birthday, North Korean media have reported a festive mood in the country, with arts performances being held as well as exhibitions in Pyongyang, the North's capital, of the Kimjongilia — a red flower cultivated to bloom around Kim's birthday.

As on other major holidays, groups of people and soldiers visited a giant statue of Kim's father in Pyongyang to offer flowers, and bow in respect or salute in the cold morning air.

"At the time of this significant February holiday I want to see President Kim Il Sung more than ever. That's why I've come to this statue early in the morning," Ri Un Ha, a North Korean woman, told Associated Press Television News.

In images broadcast on state television, North Koreans wearing Hanbok — brightly colored, loose-fitting traditional Korean costumes — and Western-style suits, gathered in Pyongyang's central Kim Il Sung square for a mass dance performance.

Kim hasn't yet publicly named a successor, prompting speculation abroad about who might eventually take the reclusive country's helm and whether Kim will designate one of his sons as the North's next leader — continuing the world's only communist dynasty.

Japanese Prime Minister Shinzo Abe, whose country has been at odds with North Korea over the abductions of Japanese citizens to be trained as spies, took the occasion of Kim's birthday to urge him to follow through on the Beijing nuclear agreement.

"In order to make it a good birthday, I hope North Korea will implement what the partners have decided at the six-party talks," Abe told reporters in Tokyo Thursday evening.

Russian President Vladimir Putin and China's Communist Party sent messages to congratulate Kim on his birthday, KCNA reported Friday.