Secretary of State Colin L. Powell asserted today that a 1988 poison gas attack that killed an estimated 5,000 Kurds in this farming town nestled in Iraq's barren northern mountains was ample evidence that former president Saddam Hussein's government possessed weapons of mass destruction and justified the U.S. decision to go to war.

In an emotional defense of the invasion of Iraq, Powell visited a mass grave site, toured a new museum commemorating the attacks and listened as Kurdish political leaders proclaimed that the Halabja massacre provided sufficient legitimacy to go to war.

"If you want evidence of the existence and the use of weapons of mass destruction, come here now to Halabja today and see it," Powell said after walking through the museum. "What happened over the intervening 15 years? Did [Hussein] suddenly lose the motivation? Did he suddenly decide that such weapons would not be useful? The international community did not believe so."

Powell's three-hour visit to this town near the Iranian border, which required him to fly in a convoy of UH-60 Black Hawk and AH-64D Apache Longbow attack helicopters, brought him face to face with scores of Iraqi Kurds who praised the U.S. invasion and held aloft signs lauding President Bush. The sentiments on display here were far more ebullient than those generally expressed by Iraqis in parts of the country Powell did not visit.

"Today it is perplexing and rather painful indeed for the people of Halabja to hear voices in the international community that continue to insist on proof for Saddam's weapons of mass destruction," said Barham Salih, the prime minister for the western part of Iraq's Kurdish region. "Here is the proof. Halabja is the proof. . . . This mass grave in Halabja and the other 170 so far discovered mass graves in Iraq should dispel any doubts about the legitimacy of the American and British liberation of Iraq. These mass graves vindicate the moral imperative of your intervention to protect the people of Iraq."

Powell was joined by Iraq's top Kurdish leaders, Massoud Barzani of the Kurdistan Democratic Party and Jalal Talabani of the Patriotic Union of Kurdistan, who preside over a swath of northern Iraq that had been autonomous since it split from Iraq after the 1991 Persian Gulf War. The two leaders, who sit on the country's interim Governing Council, expressed opinions similar to those of Salih. "The tragedy of Halabja gives them the legitimacy for going to war," Barzani said.

The attack on Halabja occurred in the waning days of Iraq's eight-year war with Iran. Furious that Kurdish militiamen in the area had allied themselves with advancing Iranians, Hussein ordered his cousin, Ali Hassan Majeed, to carry out a retribution campaign against the Kurds starting in 1987 that included forced relocations, the destruction of villages and the killing of an estimated 182,000 people.

At about noon on March 16, 1988, after two days of conventional bombing, the Iraqi air force dropped sarin, tabun, VX and mustard gases on Halabja. The toxic cloud drifted over the town, killing an estimated 5,000 people and harming 10,000. Those who survived did so by running into the hills or by hiding in their basements.

"It was such a great tragedy," said Esmail Abdulrahim Saleh, an English teacher, who said nine of his relatives died in the attack. "It is impossible to describe."

Although the United States condemned the Iraqi government's use of chemical weapons as a "grave violation" of international law, the Reagan administration did not sanction Hussein, who was regarded as a U.S. ally because of his war against Iran's Islamic revolutionary government. At the time, the State Department said there were "indications" that Iran had used chemical artillery shells against Iraqi positions in the area.

Asked today about the U.S. response, Powell, who was Reagan's national security adviser, told reporters that "there was no effort on the part of the Reagan administration to either ignore it or not take note of it." But when speaking to about 250 relatives of victims, Powell said there should have been a more aggressive response.

"I cannot tell you the world should have acted sooner," he told the relatives. "You know that."

Standing at the mass grave site in front of rows of gravestones aligned in perfect diagonals with the precision of a military cemetery, Powell said the toppling of Hussein would prevent such atrocities in the future.

"What I can tell you is that what happened here in 1988 is never going to happen again," he said, noting that Majeed, the alleged architect of the Halabja attack and better known by his nickname, "Chemical Ali," had been detained by U.S. forces.

"Chemical Ali is in jail," he told the relatives, many of whom were clutching brightly colored bouquets of silk flowers and holding framed photos of dead family members. "He will stay in jail until an Iraqi court decides his fate. Saddam is running and hiding. . . . Beyond that, the system that spawned them, a system of coups and plots and assassins, is smashed and will never return."

Powell did not meet with any of the 1,200 American specialists who are scouring Iraq for evidence that Hussein had an active banned-arms program in recent years. At a news conference Sunday, Powell said there "was no particular need" to meet with them because the director of the search effort, former U.N. weapons inspector David Kay, will soon issue a report.

After touring the museum, which contains a life-size diorama of a dozen dead villagers complete with fake gas produced with liquid nitrogen, Powell had private talks with Barzani, Talabani and other Kurdish leaders before he flew to Kuwait. The leaders pressed Powell for assurances that Kurdish areas would be given continued autonomy under a new government.

Colin L. Powell meets with residents of Halabja, a Kurdish town where Saddam Hussein's forces unleashed a variety of poisonous gases in the closing months of the Iran-Iraq war.