SEOUL, South Korea — North Korea could be planning another nuclear test for next year, a South Korean think tank warned Friday, as Pyongyang celebrated the anniversary of leader Kim Jong Il’s elevation to military chief, calling him "an invincible and iron-willed commander."

The 19th anniversary of Kim’s being named supreme commander of the Korean People’s Army came a day after South Korea held massive drills near the world’s most heavily armed border — part of a series of exercises that have angered the North and prompted it to threaten to launch a "sacred" nuclear war if attacked.

Heated rhetoric has increasingly been heard from both sides, as tensions have soared between the rivals in the month since a North Korean artillery barrage on a South Korean island killed four people.

On Friday, the North lauded Kim Jong Il and his "songun," or "military-first," policy.

"Kim Jong Il is the benevolent father and a mainstay for the faith of the army and people," Pyongyang’s main Rodong Sinmun newspaper said in an editorial carried by the North’s official Korean Central News Agency, calling Kim "an invincible and iron-willed commander" who "leads the single-minded tens of millions of ranks of revolutionary fighters with love and trust."

With relations at a low, North Korea has been struggling to restart negotiations aimed at ending its nuclear weapons program — talks that have provided it much-needed aid in exchange for pledges to disarm. The program — and two widely condemned nuclear tests — has drawn a raft of sanctions from the international community, making Pyongyang increasingly reliant on aid to feed its people.

But far from backing away from the nuclear issue, North Korea is expected to continue to work on its atomic weapons programs in 2011, a South Korean Foreign Ministry-affiliated think tank said Friday.

"The possibility is always open for a third nuclear test to improve" its atomic capabilities, the Institute of Foreign Affairs and National Security said in a report.

North Korea is believed to have enough weaponized plutonium for at least a half-dozen atomic bombs, and also has revealed a uranium enrichment program that would give it a second way to make nuclear weapons.

The soaring tensions between the rival Koreas have raised worries of more violence on the peninsula, which still remains technically at war since the 1950-53 Korean War ended in a cease-fire, not a peace treaty.

According to a report released Thursday by the International Crisis Group, the Koreas’ disputed maritime boundary and the volatility of North Korea’s internal politics have "created a serious risk that any further provocation might turn into a wider conflict." North Korea’s attack was on Yeonpyeong Island, which lies in disputed waters off the peninsula’s west coast.

While the North would lose in an all-out war against the South and the United States, "Seoul is constrained in retaliating forcefully because it has so much to lose" economically and politically, the report said.

Despite the high tensions between the Koreas, Paik Hak-soon, an analyst at the private Sejong Institute think tank near Seoul, said North Korea probably won’t provoke the South again ahead of next month’s summit between Chinese President Hu Jintao and President Barack Obama in Washington. Hu’s official state visit will take place Jan. 19.

North Korea has no reason to carry out a provocation as it wants to engage in talks with the United States, Paik said, and another attack would make the visit a very difficult one for China, Pyongyang’s only major ally and main benefactor.

Still, South Korea’s military maintains its readiness in preparation for any possible North Korean attack, the Defense Ministry said Friday, without elaborating.

In a move that will further anger the North, South Korea also announced Friday that it will extend the period that a huge steel Christmas tree near the border will stay lit.

Seoul originally planned to turn off the tree’s blinking lights, which are visible from border towns in atheist North Korea, on Sunday but will now do so Jan. 8, as it "may give a message of peace to North Korea," Defense Ministry spokesman Kim Min-seok said. North Korea sees the tree as propaganda warfare.

North Korea officially guarantees freedom of religion for its 24 million people. But authorities crack down on Christians, who are seen as a Western-influenced threat to the government. The distribution of bibles and secret prayer services can mean banishment to a labor camp or execution, defectors say.

For North Koreans, a personality cult surrounding the country’s founder Kim Il Sung and his son Kim Jong Il serves as a virtual state religion.