— In one of the world’s most volatile ­regions, Pakistan is advancing toward a sea-based missile capability and expanding its interest in tactical nuclear warheads, according to Pakistani and Western analysts.

The development of nuclear missiles that could be fired from a ship or submarine would give Pakistan “second-strike” capability if a catastrophic nuclear exchange destroyed all land-based weapons. But the acceleration of Pakistan’s nuclear and missile programs is renewing international concern about the vulnerability of those weapons in a country that is home to more than two dozen Islamist extremist groups.

“The assurances Pakistan has given the world about the safety of its nuclear program will be severely tested with short-range and sea-based systems, but they are coming,” said Michael ­Krepon, co-founder of the Stimson Center, a Washington-based global security think tank. “A cardinal principle of Pakistan’s nuclear program has been, ‘Don’t worry; we separate warheads from launchers.’ Well, that is very hard to do at sea.”

Western officials have been concerned about Pakistan’s nuclear program since it first tested an atomic device in 1998. Those fears have deepened over the past decade amid political tumult, terrorist attacks and tensions with the country’s nuclear-armed neighbor, India, with which it has fought three wars.

That instability was underscored this month as ­anti-
government protests in the capital appeared to push Prime Minister Nawaz Sharif’s government to the brink of collapse. The political crisis was unfolding as Pakistan and India continued lobbing artillery shells across their border, in a tit-for-tat escalation that illustrated the continued risk of another war.

For more than a decade, Pakistan has sent signals that it is attempting to bolster its nuclear arsenal with “tactical” weapons — short-range missiles that carry a smaller warhead and are easier to transport.

Over the past two years, Pakistan has conducted at least eight tests of various land-based ballistic or cruise missiles that it says are capable of delivering nuclear warheads. Last September, Sha­rif, citing “evolving security dynamics in South Asia,” said Pakistan is developing “a full-
spectrum deterrence capability to deter all forms of aggression.”

The next step of Pakistan’s strategy includes an effort to develop nuclear warheads suitable for deployment from the Indian Ocean, either from warships or from one of the country’s five diesel-powered submarines, analysts say. In a sign of that ambition, Pakistan in 2012 created the Naval Strategic Force command, which is similar to the commands in the air force and army that oversee nuclear weapons.

“We are on our way, and my own hunch is within a year or so, we should be developing our second-strike capability,” said Shireen M. Mazari, a nuclear expert and the former director of the Institute of Strategic Studies Islamabad, a hawkish Pakistani-government-funded think tank.

Pakistan’s nuclear push comes amid heightened tension with U.S. intelligence and congressional officials over the security of the country’s nuclear weapons and materials. The Washington Post reported in September 2013 that U.S. intelligence officials had increased surveillance of Pakistan in part because of concerns that nuclear materials could fall into the hands of terrorists.

State Department spokeswoman Jen Psaki, asked whether the United States is concerned about a sea-launched Pakistani weapon, said it is up to Pakistan to discuss its programs and plans. But, she said, “we continue to urge all nuclear-capable states to exercise restraint regarding nuclear and missile capabilities. We continue to encourage efforts to promote confidence-building and stability and discourage actions that might destabilize the region.”

During a visit to Washington for consultations with the Obama administration in July, Tariq Fatemi, Sharif’s senior foreign policy adviser, said the government had “no intention of pursuing” sea-based nuclear weapons.

It is unclear how much direct knowledge Sharif’s government has about the country’s nuclear weapons and missile-development programs, which are controlled by the powerful military’s Strategic Planning Directorate. But the prime minister is the chairman of the country’s National Command Authority, a group of civilian and military officials who would decide whether to launch a nuclear weapon.

Pakistani military officials declined to comment on the nuclear program. They note, however, that a January report by the Washington-based Nuclear Threat Initiative named Pakistan the “most improved” in safeguarding nuclear materials.

Analysts say much about Pakistan’s program remains a mystery. Western experts, for example, are divided over whether Pakistan has the ability to shrink warheads enough for use with tactical or sea-launched weapons.

“They may have done so, but I can’t imagine it’s very reliable,” said Jeffrey Lewis, a nuclear and nonproliferation scholar at the Monterey Institute of International Studies. Still, Lewis and other analysts say Pakistan is without doubt embarking on an ambitious multi-year strategy to enhance its nuclear arsenal and delivery systems.

In 2011, nongovernment experts interviewed by The Post estimated that Pakistan had built more than 100 deployed nuclear weapons. Now Pakistan’s fourth plutonium-production reactor is also nearing completion, and while most assessments of the country’s warhead inventory have not changed much in recent years, analysts say Pakistan continues to produce weapons material and develop delivery vehicles, positioning itself for another spurt of rapid growth at any time.

“They are going to make as much fissile material as they possibly can and keep making as many warheads as they possibly can,” said Pervez Hoodbhoy, a leading Pakistani nuclear expert and physicist.

India, which experts estimate has 80 to 100 deployed nuclear weapons, has a stated policy of using them only in response to an attack. Pakistan has repeatedly declined to embrace a no-first-use policy.

But concerns within Pakistan about India’s growing nuclear ambitions are helping to fuel Pakistan’s own advancements.

India, too, has been stepping up research and development of offensive and defensive weapons systems. In 2012, India test-launched its first intercontinental ballistic missile, which it said has a range of more than 3,100 miles. In February, the Times of India reported that the missile, as well as the country’s first nuclear-powered submarine, could be deployed as early as next year. In May, India conducted its first test of a planned missile defense system.

Much of India’s ballistic technology appears aimed at boosting its defenses against China, not Pakistan. But the Pakistani military has been shifting the focus of the country’s nuclear program over the past decade because of fears that Indian forces could use the threat of terrorism to launch a sudden cross-
border strike.

India has a sizable advantage in conventional weapons, and its army is more than twice the size of Pakistan’s. And in recent years, Pakistan’s army says, more than one-third of Pakistan’s 500,000 soldiers have been focused not on the eastern frontier, but on battling Islamist militants on the region bordering Afghanistan.

So instead of working to enhance the range of its missiles, Pakistan is developing shorter-range cruise missiles that fly lower to the ground and can evade ballistic missile defenses, analysts say.

Pakistan has repeatedly tested its indigenously produced, nuclear-capable Babur cruise missile, which has a range of 400 miles and can strike targets at land and sea, military officials said. In 2011 and last year, Pakistan also tested a new tactical, nuclear-capable battlefield missile that has a range of just 37 miles.

“This is the miniaturization of warheads,” said Mansoor Ahmed, a strategic studies and nuclear expert at Quaid-i-Azam University in Islamabad.

Maria Sultan, chairwoman of the Islamabad-based South Asian Strategic Stability Institute, an organization with close links to Pakistani military and intelligence officials, said the short-range missile is designed as a signal to India’s military.

“We are saying, ‘We have target acquisition for very small targets as well, so it’s really not a great idea to come attack us,’ ” Sultan said. “Before, we only had big weapons, so there was a gap in our deterrence, which is why we have gone for tactical nuclear weapons and cruise missiles.”

Still, even a limited use of nuclear weapons on the battlefield would likely trigger a major retaliatory strike from India, said Manpreet Sethi, a senior fellow at the New Delhi-based Centre for Air Power Studies.

“The use of tactical nuclear weapons is not going to change an [Indian] offensive in any substantial way,” Sethi said. “Slow down, yes, but not stop.”

Hans Kristensen, director of the Nuclear Information Project at the Federation of American Scientists, said the fact that Pakistani and Indian analysts even debate the outcome of a limited nuclear exchange is cause for alarm.

“India and Pakistan have so many avenues into a conflict that could spin out of control,” Kristensen said. “The development of these weapons systems lowers the point where you could potentially see nuclear weapons come into use.”