(Updates after news conference)

By Stephanie Nebehay

GENEVA, Aug 7 (Reuters) - Russia and the United States will need to make major efforts to reach a 2012 deadline for destroying their huge stockpiles of chemical weapons, the head of a treaty verification body said on Tuesday.

Substantial amounts of "some of the most toxic and dangerous substances ever invented" remain in the two countries, said Rogelio Pfirter, director-general of the Organisation for the Prohibition of Chemical Weapons.

Russia has destroyed 22 percent of its stockpile and the United States 46 percent in the past decade.

"Political will appears to be there so I have to believe they are on track. But it will require a major effort on their part, no question about that because the percentages which are still to be destroyed are very, very important," Pfirter said.

He was addressing the United Nations-sponsored Conference on Disarmament, which negotiated the 1997 Chemical Weapons Convention banning the use, development, production, stockpiling and transfer of chemical weapons.

Inspectors have certified that nearly 24,000 metric tonnes of chemical agents -- or 33 percent of declared stockpiles worldwide -- have been destroyed under the landmark pact.

Russia and the United States, which accounted for 67,000 metric tonnes of the 70,000 metric tonnes of chemical weapons declared by six countries, were granted a five-year extension from the original 2007 deadline for destroying their stockpiles.

The treaty, which entered into force in April 1997, has been ratified by 182 countries. Thirteen countries have shunned it -- including Israel, Egypt, Syria, North Korea and Myanmar.

NERVE AGENTS

But destroying chemical weapons is expensive and requires sophisticated technology, according to Pfirter.

Referring to work underway at a destruction facility in Maradykovsky, Russia, he said: "These nerve agents are weaponised in rockets. So you have not just to neutralise the agent, you have to ensure there is no explosion."

The United States, which has spent $20 billion on destroying stockpiles, estimates it will cost another $40 billion to finish the job, according to Pfirter.

Among the other four declared holders, Albania last month became the first to confirm destruction of its entire chemical weapons stockpile, which included mustard gas and other agents.

Pfirter strongly regretted that the Middle East region has largely opted out of the pact, although he said both Iraq and Lebanon had signalled that they will join soon.

"The lack of support for the convention in the Middle East represents a serious void on our map, where Egypt, Israel and Syria continue to cite regional security concerns for not joining the convention," he told the Geneva arms control forum.

He later told reporters: "The fact is that the practical result of not coming into the Chemical Weapons Convention ... is that the peoples of the Middle East are still subject to the possibility of chemical weapons being used there against them."

Chlorine truck bombs used to kill and maim civilians in Iraq "serve as a stark reminder of the dangers that the misuse of toxic chemicals, even the most common ones, pose to our security," Pfirter said.

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