WASHINGTON (AP) — It's not easy or quick to get rid of a nation's chemical weapons. Just ask the United States.

Three decades after the U.S. started destroying its own chemical weapons, the nation's stockpile stands at more than 3,000 tons — about three times what the U.S. now says Syrian President Bashar Assad controls.

While the U.S. has made significant progress eradicating 90 percent of the 31,500 tons it once possessed, the military doesn't expect to complete destruction until 2023.

Experts say it's probably simpler to make chemical weapons than to get rid of them.

"Disposal requires such rigorous processes to ensure there is no pollution or residual agent," said Susannah Sirkin, international policy director for Physicians for Human Rights, which has been monitoring weapons of mass destruction for more than two decades. "On average it is costing about 10 times more to destroy than it did to make the munitions."

The two basic destruction methods — chemical neutralization and incineration — both require specialized facilities. Using incineration, chemicals must be heated to thousands of degrees. Decades-old storage containers can be leaky and tough to handle. And destruction produces highly hazardous waste that must be carefully stored.

Assembled weapons, where chemicals already have been loaded into rockets and packed with explosives, pose their own dangers. They also can leak, or go off by accident, contaminating the environment.

Sirkin said negotiators of the Chemical Weapons Convention that banned such weaponry probably thought the initial 10-year deadline for the U.S. to dispose of everything sounded reasonable. Engineers would have told them differently; especially when it takes time to build the facilities and to overcome not-in-my-backyard objections from people who live near the plants.

Now, as the U.S. and others push Syria to surrender its arsenal, the steep challenges that have hindered America's efforts for a generation illustrate the daunting task of securing and ultimately dismantling Assad's stockpiles in the middle of a civil war.

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Secretary of State John Kerry and Russian Foreign Minister Sergey Lavrov were meeting over two days with chemical weapons experts in Geneva to discuss how to do exactly that. They hoped to emerge with the outlines of a plan.

The remaining U.S. stockpile includes many of the same chemicals in Assad's possession. The Syrian regime has more than 1,000 tons of sulfur, mustard gas and the ingredients for sarin and the nerve agent VX, Kerry told Congress this week.

Under a tenuous diplomatic deal being coordinated by Russia, which holds the world's largest remaining chemical weapons stockpile, Syria would join the Chemical Weapons Convention, declare its stockpiles and hand them over to the international community for destruction, all to avert a punitive U.S. military strike.

It's unclear how that colossal task could be carried out when there's distrust of Syria in the international community, uncertainty about the weapons' locations and ongoing fighting between Assad's forces and rebels. The White House says it will require extensive verification to ensure that stall tactics aren't disguised as legitimate holdups.

In the U.S., those holdups have ranged from environmental delays and political opposition to technical and safety challenges to tough laws restricting the transport of chemical weapons. Likewise, it's been difficult to round up the tens of billions of dollars to pay for destroying the cache.

"All of this is a slow process," said Dieter Rothbacher, a former U.N. chemical weapons inspector who has worked in Iraq, Russia and the U.S. "Falling behind (schedule) is actually relatively easy."

The U.S. started developing chemical weapons around World War I, steadily increasing its capabilities through World War II until 1968. The stockpile grew to about 31,500 tons of sarin, VX, mustard gas and other agents, according to the Army. Russia, by comparison, has said it amassed about 44,000 tons.

The move toward destroying the United States' chemical weapons started in the 1970s, building momentum in the 1980s when Congress directed the Defense Department to start eliminating the stockpile.

That commitment became an international obligation when the U.S. signed the Chemical Weapons Convention in 1993 and ratified it four years later. That started the clock on a 10-year period in which the U.S. was supposed to destroy the rest of its chemical weapons.

The Army used to destroy chemical weapons at nine sites across the country. By January 2012, troops had completed 90 percent of the job, and only two active sites now remain.

In an arid stretch of desert about 40 miles south of Colorado Springs, Colo., sits the Pueblo Chemical Depot. The Army says that's where about 2,600 tons of mustard gas is situated in projectiles and mortar cartridges. The destruction facility is finished but still being tested, with plans to start operations in 2015. The depot employs more than 900 people and is expected to end its work by 2019.

The other site, just outside Richmond, Ky., isn't as far along. The destruction plant at the Blue Grass Army Depot is only about 70 percent complete as of this summer, and the Army doesn't expect to open it until 2020. Work at Blue Grass to destroy 523 tons of nerve and blister agents stored in rockets and projectiles should wrap up by 2023, if everything goes as planned.

The U.S. has long since missed its original 2007 deadline, which was extended to 2012, then missed again. Russia is behind schedule too.

Such are the odds as the U.S. and its allies turn to Syria and demand swift, complete and verifiable action to ensure never again can Assad use poison gas.

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Follow Josh Lederman on Twitter at http://twitter.com/joshledermanAP