WASHINGTON (Reuters) - This week's deadly terrorist attacks could be followed by devastating assaults involving biological, chemical or even nuclear weapons, U.S. lawmakers and experts said on Saturday.

``In my judgement, it's not a question of if there will be a biological or chemical weapons attacks, but when -- and of what magnitude,'' said Rep. Christopher Shays, the Connecticut Republican who heads the House Government Reform subcommittee on national security.

Attacks involving nuclear weapons were less likely, but remained a possibility, Shays told Reuters.

Shays, whose committee has held 17 hearings on terror threats, said the attacks on the World Trade Center and the Pentagon, which may have killed over 5,000 people, underscored the need to step up efforts to combat terrorism.

``The bottom line is, a chemical, biological or nuclear attack by a terrorist is a very real possibility,'' Shays said. ''It sends shivers down your back.''

British Foreign Secretary Jack Straw on Friday urged immediate attention to the ``next threat to our collective security,'' noting that the people responsible for Tuesday's attacks would stop at nothing.

``It should now be obvious to everyone that people who have the fanaticism and capability to fly an airliner laden with passengers and fuel into a skyscraper will not be deterred by human decency from deploying chemical and biological weapons, missiles or nuclear weapons or other forms of mass destruction, if these are available to them,'' Straw said.

Over the past few years, U.S. government officials and independent experts have urged increased efforts to brace for any such attack -- warning that the United States is currently ill-prepared to deal with terrorist threats.

Air Force Gen. Richard Myers, newly confirmed chairman of the U.S. Joint Chiefs of Staff, last week told U.S. senators the U.S. could be ``vulnerable'' to a rogue attack with biological weapons.

``I think it's a recognized shortfall ... the ability to combat weapons of mass destruction to include chemical and biological,'' Myers said during his confirmation hearing.

DIFFICULT TO DEFEND

Former Sen. Sam Nunn this month told lawmakers a simulated terror attack involving a release of smallpox showed how difficult it would be to defend against such an assault.

The exercise ended with more than 1,000 people dead and 15,000 reported smallpox cases -- all simulated -- less than two weeks after 24 ``patients'' first showed signs of an undiagnosed illness at an Oklahoma hospital. The simulation ended with no resolution to the ``epidemic.''

Smallpox is a highly contagious disease last seen in the United States in 1949. Vaccination ceased in 1972, leaving current generations of Americans with no immunity.

Nunn, a Georgia Democrat, told a Senate Armed Services committee hearing Sept. 5 that smallpox killed more than 300 million people in the 20th century, more than those killed in all the wars of the century combined.

A group of independent experts, headed by former senators Warren Rudman and Gary Hart, have released three reports over the past three years identifying terrorism as the most urgent national security issue facing the United States.

The group's third and final report, released in January, recommended creation of a National Homeland Security Agency -- with a seat on the Cabinet -- and an overhaul of national security priorities.

Vice President Dick Cheney is leading an administration working group to assess terrorist threats and is expected to report the findings to Congress by October 1.

Recognizing the seriousness of the bioterrorism threat, the United States is producing small amounts of chemical and biological warfare agents, including one for a deadly new form of anthrax, in order to develop protection against them.

``The threat is real. It is growing, and it is the responsibility of the United States military and this administration to protect us against it,'' Defense Department spokeswoman Victoria Clarke told reporters earlier this month.

She said no actual agents had been produced yet in the ''defensive'' program, which has been going on for at least four years, but there were plans to develop agents to cause such diseases as a new and virulent strain of anthrax within the restrictions of the global Biological Weapons Convention.

That 1972 treaty, signed by the United States, bans nations from developing or acquiring weapons that spread disease, but it allows work on vaccines and other protective measures.

TERRORISTS COULD HAVE ACCESS

Joseph Cirincione at the Carnegie Endowment for International Peace, said there was no hard evidence that Saudi exile Osama bin Laden, a prime suspect in the attacks, had acquired nuclear, biological or chemical weapons.

But he said bin Laden, who has been living in Afghanistan, could easily buy such weapons from corrupt officials in Russia, or steal them from poorly guarded facilities, where some 20,000 battlefield nuclear weapons are stored.

Alternately, he could also acquire chemical or biological agents through cooperation with Iraq, Cirincione said.

``It's certainly possible,'' Cirincione said. ``Do I think it's likely? No, but it is possible.''

Sen. Joseph Biden, the Delaware Democrat who heads the Senate Foreign Relations Committee, told CNN's ``Larry King Live'' the United States should increase its focus on gathering intelligence as part of a major effort to battle terrorism.

``I think this is a heck of a wake-up call,'' Biden said.

He lauded President Bush's effort to build a global coalition to fight terrorism.

``This is an opportunity to begin the end of international terrorist networks. We can't do it alone, we've got to do it with the help of other countries,'' Biden said.