SEOUL — North Korea pledged today to detail its nuclear programs and disable all activities at its main reactor complex by the end of the year, its firmest commitment to disarm after decades seeking to develop the world’s deadliest weapons.

The agreement at talks in China came on the same day the North Korean leader, Kim Jong Il, held talks in the communist nation’s capital of Pyongyang with President Roh of South Korea at the first summit between the two countries in seven years.

In Washington, President Bush hailed the nuclear deal and said it reflected the “common commitment” of the talks to shut down North Korea’s atomic weapons program.

Under an agreement reached in February, Pyongyang was required to shut down and seal its sole operating reactor at its main nuclear complex, which it did in July after America reversed its hard-line policy against the regime. The second phase required it to disable the reactor and provide a full description of all its nuclear programs. Today’s agreement calls for that to happen by the end of the year.

The North said it would allow America to lead a group of experts to Pyongyang within two weeks “to prepare for disablement” of its nuclear facilities, a vice foreign minister of China, Wu Dawei, said in Beijing. America wants the dismantling process to be so thorough that a nuclear facility could not be made operational for at least 12 months.

America was secretive about what it promised in return.

America has agreed to lead disablement activities and provide the initial funding for them. Washington also reiterated its willingness to remove North Korea from a list of countries that sponsor terrorism, a key demand of Pyongyang.

No timetable was set for this action, but a joint statement said it will happen “in parallel with” the North Korean government following through on its commitment.

“The two sides will increase bilateral exchanges and enhance mutual trust,” the statement says.

Besides America and China, three other countries — Russia, South Korea, and Japan — participated in the talks with the North.

The five countries reiterated a commitment to deliver aid under the February disarmament deal granting the North the equivalent of 1 million tons of fuel oil. On Friday, in anticipation of the new agreement, America also announced it would spend up to $25 million to pay for 50,000 tons of heavy fuel oil for North Korea.

This is not the first nuclear deal Washington has struck with the reclusive, communist regime. In a previous American attempt to halt the North’s nuclear weapons development, Pyongyang simply froze its arms programs under a 1994 deal.

The latest nuclear standoff began in 2002, after Washington accused the North of a secret uranium enrichment program — in addition to its known plutonium-based facilities. Either material enriched to a sufficient extent can be used to make bombs.

The North then quickly restarted production of weapons-grade plutonium at its main nuclear complex, leading to its first-ever test nuclear explosion in October 2006. Experts say the North may have produced more than a dozen nuclear bombs.

Today’s agreement commits the North to make a “complete and correct declaration of all its nuclear programs” — which America has said would include the uranium issue.

There was no immediate comment on the nuclear deal from Pyongyang, where leaders of the two Koreas met for four hours to further reconciliation efforts.

The summit in the North’s capital was the second ever between leaders of the two countries, which remain technically at war since the Korean War ended in a 1953 cease-fire that has never been replaced with a peace treaty.