WASHINGTON -- With his sudden agreement to the first North-South summit, North Korean President Kim Il Sung has expanded his chances of wringing concessions from the West in return for ultimately abandoning a nuclear arsenal.

The proposal for a meeting between North and South Korean presidents, the first since the peninsula was divided in 1945, was widely hailed in South Korea as a breakthrough in the nuclear standoff.

North Korean President Kim Il Sung proposed the summit through former U.S. President Jimmy Carter, who returned to Seoul yesterday after an unofficial diplomatic mission to North Korea and will brief White House officials in Washington this morning.

From the U.S. standpoint, however, both the agreement and the whole Carter mission are a mixed bag. While helping to defuse a brewing crisis, they may undermine the Clinton administration's effort to bring international pressure to bear on North Korea and may strengthen the Communist regime's bargaining power.

Already, as a result of its actions in recent weeks, North Korea had put itself in a stronger position to seek aid and other concessions from the United States and its allies.

hastily discharging fuel from its five-megawatt nuclear reactor, the Communist regime had effectively placed in storage for future development enough fuel for up to a half-dozen atomic bombs, experts say, on top of the one or two it may already have. This means that, even if North Korea "freezes" its nuclear activity during negotiations, it could break out of international safeguards and restart its program at a more advanced stage if talks turn sour.

"It's a whole new ballgame on the ground if this material is reprocessed and put into weapons," said Jon Wolfsthal of the independent Arms Control Association in Washington.

Now, with the summit offer, relayed through Mr. Carter and immediately accepted by the South, Mr. Kim has added leverage: He could weaken the alliance of the countries pressuring his -- the United States, South Korea and Japan.

For months, North Korea resisted any dealings with the South on its nuclear program, preferring to deal directly with the United States. Eventually, South Korea succumbed, and dropped a demand for North-South talks. It also, after months of frustration with the North, finally agreed to seek phased United Nations sanctions against North Korea.

Now, North Korea has given what the South has long sought. This explains South Korean optimism yesterday, though past summit proposals have failed.

Even if a summit occurs, Mr. Wolfsthal said that he doubts this will divide South Korea from the United States. But other experts say it could at least be a tactic to stall the U.S.-led sanctions initiative.

It could exploit what Richard Haass, a Bush administration national security council staffer, describes at divergent U.S. and South Korean perspectives. South Korea sees the nuclear threat from the North purely as a threat to itself and the Asian region. The United States looks beyond that to a weakening of global nuclear controls and the eventual possibility that North Korea could sell weapons and technology to terrorist states in the Middle East.

"This is a perfect way to distract attention from the real issue, which is North Korea's nuclear activities," Mr. Haass said.

Summits provide "an aura of progress," and this could "too easily tactical" on North Korea's part. Perhaps with these dangers in mind, the White House was extremely circumspect in reacting to yesterday's developments.

Mr. Carter will meet at 10 a.m. today with National Security Adviser Anthony Lake. Although no plans were disclosed for a meeting between Mr. Carter and President Clinton, who is at Camp David, the president has already said he wants to talk to Mr. Carter on his return.

Of the North-South summit, a White House official would only say, "We welcome [South Korean President] Kim Young Sam's initiative to resume a North-South dialogue. It's a bilateral issue. It's been a Kim Young Sam initiative for some time.

"All of the North Korean assurances that have been reported by President Carter will be explored through diplomatic channels," the White House official went on, noting North Korea's past tendency to backtrack on understandings reached with the United States and nuclear inspectors. Even if what Mr. Carter said is confirmed through lower-level contacts, "we will have to analyze carefully the clarity of the confirmation," the official said.

With his harsh criticism of sanctions yesterday, Mr. Carter may have further weakened chances for building international pressure on North Korea. At a news conference in Seoul, the former president said, "In my opinion, the pursuit of sanctions is counterproductive in this particular and unique society."

President Clinton said Friday that the United States will keep pushing for sanctions unless North Korea accepts U.S. terms for a resumption of negotiations. Two rounds of talks were held last year; a planned third round was called off by the United States when North Korea flouted international nuclear controls.

The Carter mission, however, may have allowed the administration to pull back from a confrontation that it wasn't yet prepared for.

The administration had already failed to win support for sanctions either from China or from Russia, both of which hold Security Council vetoes. And it was not in a strong position anyway to enlist international support for tough action against North Korea, since the North has not yet moved ahead to expand its nuclear program.

All the North has done so far is to hide evidence of its past weapons development, and Mr. Wolfsthal and other experts say this evidence eventually might be recovered. For now, inspectors are on hand to make sure the discharged fuel, once cooled, isn't turned into weapons fuel.

While pressing at the United Nations for sanctions, the United States hadn't yet moved ahead to bolster strongly its military forces in the South in case North Korea retaliated militarily

against the new outside pressure. The Pentagon has reportedly prepared a series of recommendations for speedily reinforcing U.S. troops in South Korea in the event of attack.

Hawkish experts outside the administration say it should be laying the military groundwork now for the eventual possibility that North Korea will break out of international safeguards and start to expand its nuclear program. If the North starts to refuel the reactor, reprocess the discharged fuel, turn fuel into actual weapons, or operate its planned larger reactor, the United States should be ready to take military action, argues Mr. Haass.

The Associated Press contributed to this article.