If Iran is allowed to pursue a nuclear weapons capability, countries around the world might feel compelled to take the same path, Brent Scowcroft, who advised two U.S. presidents on national security, said on Thursday.

"We're on the cusp of an explosion of proliferation and Iran is now the poster child," said Scowcroft, who served under former President Gerald Ford and President George Bush, the father of former President George W. Bush.

"If Iran is allowed to go forward, in self-defense or for a variety of reasons we could have half-a-dozen countries in the region and 20 or 30 more around the world, doing the same thing, just in case," he told the Senate Foreign Relations Committee.

But Scowcroft and another former U.S. national security adviser, Zbigniew Brzezinski, counseled against using military force to try to prevent Iran from developing nuclear weapons.

They said negotiations with Tehran should be pursued instead.

As they testified, U.S. Secretary of State Hillary Clinton announced in Brussels that the Obama administration intends to invite Iran to an international conference on Afghanistan planned for this month.

President Barack Obama says he wants to engage Tehran, a dramatic turnaround from the policy of former President George W. Bush that sought to isolate the Islamic republic.

The United States and some of its allies fear that Iran is seeking nuclear weapons. Iran says its sensitive nuclear work is for civilian power purposes, not an atomic bomb.

Saudi Arabia, Egypt and Turkey were among the countries that might want to keep up with Iran if it continues to enrich uranium, Scowcroft said. Enriched uranium can be used as fuel for power plants, and provide material for nuclear bombs if enriched much more.

Israel, an enemy of Iran's Islamic government, is already widely believed to have an arsenal of nuclear weapons.

Brzezinski, who served under former President Jimmy Carter, agreed that proliferation may be the greater threat from Iran's nuclear programs, rather than the danger that Tehran would make the "suicidal" move of firing the first bomb it built.

Talks were preferable to a military conflict with Iran, which would "absolutely devastate the historical legacy of the Obama administration," Brzezinski said.

Scowcroft said the United States should work more closely with five powers that have joined with it in previous talks with Iran - Britain, France, Germany, Russia and China - so that Iran faces "a monolith that says, 'Don't do this'."