The nuclear ballistic missiles that have deterred threats since the Cold War no longer are adequate for today's fight against terrorist groups, the former leader of U.S. Strategic Command said Tuesday.

The U.S. needs to have long-range conventional weapons that mirror its nuclear ballistic missiles, retired Air Force Gen. Robert Kehler told the House Armed Services subcommittee on strategic forces.

"Violent extremists and nation states are not the same, and we cannot deal with any of them in a one-size-fits-all manner," he said. "Deterrence strategies that are the preferred approaches to counter nation states will likely not be effective against violent extremists, where direct action is often the only recourse. Nuclear weapons may not be the most credible deterrence tool against some targets, against some scenarios where they were once the preferred option."

Instead, Kehler advocated for long-range conventional weapons that could carry out the same mission of hitting targets, but without the nuclear fallout.

"Today the only prompt global strike capability to engage potentially time-sensitive, leading targets continues to be a ballistic missile system armed with nuclear weapons," Kehler said.

During the Cold War, the threat of nuclear strikes and mutually assured destruction was enough for deterrence, Kehler said. But with the rise of non-state actors such as terrorist groups, the U.S. needs more options.

"Violent extremists and nation states are not the same, and we cannot deal with any of them in a one-size-fits-all manner," he said. "Deterrence strategies that are the preferred approaches to counter nation states will likely not be effective against violent extremists, where direct action is often the only recourse. Nuclear weapons may not be the most credible deterrence tool against some targets, against some scenarios where they were once the preferred option."

Instead, Kehler advocated for long-range conventional weapons that could carry out the same mission of hitting targets, but without the nuclear fallout.

"We continue to require and deploy conventional prompt strike capabilities to provide the president a range of flexible military options to address a small number of high-value targets, including in an anti-access and aerial denial environment," he said.

"In my view, such a capability would both enhance strategic deterrence and improve our ability to react quickly in a time-critical scenario, by providing the president with an option to promptly deliver a non-nuclear weapon against a limited but vitally important target or subset of high-value targets at long range," Kehler continued.

Some of those targets could be in terrorist areas or rogue nations, and might include highly defended or mobile targets in difficult to access geographic areas.

Kehler said he believes it's an important capability to have to address a rapidly changing security situation.

"I can't recall a time during my professional career when potential threats to our homeland were more varied or pronounced then they are today," he said.

The general goal of Conventional Prompt Global Strike or CPGS is to deliver a conventional payload missile anywhere in the world within one hour.

Air Force scientists have been developing technologies to allow rapidly deployed missiles, including hypersonic engines that could propel a warhead past speeds of Mach 5 – fast enough to traverse the continental U.S. in about half an hour.

An October report from the Congressional Research Service noted that the Air Force may receive $78.8 million in 2016 for continued development of a hypersonic "conventional strike missile."

"This capability may bolster U.S. efforts to deter and defeat adversaries by allowing the United States to attack high-value targets or 'fleeting targets' at the start of or during a conflict," the report said. "CPGS weapons would not substitute for nuclear weapons, but would supplement U.S. conventional capabilities. They would provide a 'niche' capability, with a small number of weapons directed against select, critical targets."

But Dr. James Acton, senior associate for the Carnegie Endowment, cautioned that the Pentagon has some decisions to make before deploying this type of missile.

"The Pentagon has no official policy that sets out the specific military missions where CPGS weapons might be required," he said.

The goal of hitting any target on Earth within an hour is a description of what the technology can do, but not what operations it might be used for, Acton said.

"Until the Department of Defense specifies these missions, there can be no yardstick against which to judge the likely effectiveness of different CPGS technologies," he said.

Use of the weapons could also lead to dangerous escalation with China or Russia due to a misunderstanding, Acton said.

The CRS report echoed the concern, noting that "some analysts, however, have raised concerns about the possibility that U.S. adversaries might misinterpret the launch of a missile with conventional warheads and conclude that the missiles carry nuclear weapons."

The report continued by noting there could be no way for U.S. adversaries to know whether a missile was armed with a conventional or nuclear payload, and therefore they might launch a retaliatory nuclear strike against the U.S.