He's the closest thing to a smoking gun in Washington's intelligence arsenal, a man who could finally and definitively link Saddam Hussein with the world's most notorious terrorist and push reluctant allies to support a U.S.-led war against Iraq.

Abu Musab Zarqawi has been linked to the murder of a U.S. diplomat in Jordan and poison plots in a half-dozen European countries. On Wednesday, Secretary of State Colin Powell told the U.N. Security Council that Usama bin Laden's man in Iraq sits atop a "sinister nexus" of terror, and that Saddam has been harboring his nefarious group.

"We are not surprised that Iraq is harboring Zarqawi and his subordinates," Powell said. "Ambition and hatred are enough to bring Iraq and Al Qaeda together."

Powell made the point to the world body that the Al Qaeda operative -- known for his expertise at concocting poison and his fanatic zeal for terrorism -- was a threat to Europe, as well as America and the rest of the world. Some 116 operatives of the cell have been arrested around the world, Powell said.

"Zarqawi and his network have plotted terrorist actions against countries including France, Britain, Spain, Italy, Germany and Russia," he said.

Not all observers were convinced of an Iraqi-Al Qaeda connection, however.

An expert on counterterrorism at the International Institute for Strategic Studies in London, Jonathan Stevenson, called the link to Zarqawi "significant," but noted that Powell failed to tie Iraq to any previous Al Qaeda operation.

France's top terrorism investigator, Judge Jean-Louis Bruguiere, said he was unaware of any direct link between Iraq and Al Qaeda. A leading investigator with Italy's anti-terrorism police said he knew of no plots by al-Zarqawi or his network in Italy.

Powell and other U.S. officials say Zarqawi has set up a camp in a corner of northern Iraq controlled by a tiny Kurdish extremist group, Ansar al-Islam. The region is outside Saddam's control, but Powell said the Iraqi president has an agent in the top leadership of Ansar al-Islam, and implied the group would not have offered Al Qaeda refuge without Saddam's consent.

In May 2002, Zarqawi received medical treatment in Baghdad after he fled Afghanistan, Powell said. He said that while Zarqawi was in the Iraqi capital, nearly two dozen other extremists converged there to establish a base of operations.

"These Al Qaeda affiliates, based in Baghdad, now coordinate the movement of people, money and supplies into and throughout Iraq for his network, and they've now been operating freely in the capital for more than eight months," Powell said. "From his terrorist network in Iraq, Zarqawi can direct his network in the Middle East and beyond."

Powell also said Iraq's embassy in Pakistan served as a liaison between Al Qaeda and the Iraqi leadership from the late 1990s until 2001, but he did not go into detail. Bin Laden was a guest at the time of the Taliban regime in neighboring Afghanistan, and Pakistan was one of just three countries to recognize the Taliban.

The secretary of state's comments were quickly rejected -- both by the Iraqi regime and the leadership of Ansar al-Islam.

"Neither I nor anybody in our group has ever seen or met al-Zarqawi, and he has never visited our area," Ansar al-Islam leader Mullah Krekar said at a news conference in Oslo, Norway. Krekar also denied that Saddam held any influence with the group, saying the Iraqi leader is a bad Muslim and "an enemy of me and my people."

Krekar watched Powell's speech on television in a restaurant in Oslo with a mobile phone cupped to his ear. He looked calm until Powell mentioned his group. "The evidence they have is weak," he said.

And in Baghdad, Iraqi presidential adviser Lt. Gen. Amir al-Saadi said: "We have no relationship with Al Qaeda."

Zarqawi, a Palestinian born in Jordan, has been on the world's intelligence radar since 1999, when he allegedly helped orchestrate a failed plot to attack American and Israeli tourists in Jordan using deadly gas. He also is being investigated in Germany for allegedly ordering attacks on Jewish and Israeli targets.

In December, Jordanian authorities identified Zarqawi as having orchestrated the slaying of Laurence Foley, a 60-year-old administrator of U.S. aid programs in Jordan. Foley was killed Oct. 28 outside his home in Amman.

Zarqawi is said to have gained his talent for manufacturing poison -- including the ultra-lethal ricin -- at military camps in Afghanistan run by bin Laden.

The poison surfaced in Britain earlier this year, after authorities arrested a group of Algerians suspected of terrorism. Police found traces of ricin at one of the suspect's apartments.

A senior administration official said recently that U.S. intelligence has indications of connections between the Algerians arrested in Britain and the Ansar al-Islam group, and officials are looking into reports that Zarqawi is also linked to the arrested men.

On Wednesday, Spanish Prime Minister Jose Maria Aznar mentioned Zarqawi in a speech to parliament, saying some of Zarqawi's alleged associates had been arrested in Spain and Britain since the Sept. 11 attacks. "The problem affects us, and close up," Aznar said.

It will be up to the world's leaders to decide how strong a case the United States made, but Powell laid out a frightening vision of what bin Laden's trained killers could accomplish with Saddam's support.

"Less than a pinch of ricin, eating just this amount in your food, would cause shock followed by circulatory failure," he told the council, adding later, "The nexus of poisons and terror is new. The nexus of Iraq and terror is old. The combination is lethal."