By Jon Hemming

KABUL, Oct 8 (Reuters) - A U.S. government scientist has met Afghan officials to ease concerns over a herbicide that can be used to spray opium crops from the air, officials said on Monday, a policy the Afghan government has so far resisted.

The Afghan government is under pressure to do more to tackle opium production after another record harvest in 2007. Afghanistan now makes 93 percent of the world's opium, with more land cultivating drugs than Colombia, Bolivia and Peru combined.

The United States has advocated eradication of opium fields, many of them in Taliban rebel-held areas, using chemical herbicides sprayed from the air, but the Afghan government has rejected the idea, citing health concerns for residents.

Dr. James Helling, senior scientific advisor to the U.S. Bureau of International Narcotics and Law Enforcement, met officials from the Afghan ministries of counter-narcotics, health, and rural rehabilitation on Sunday, officials said.

His aim was to explain how glyphosate -- the herbicide used to destroy crops -- works, not to recommend it be used, a U.S. embassy official said.

"He is here to explain what it is and how it works. He is here to discuss the science of glyphosate, not to persuade anyone they should be spraying from planes or anything," said the official who declined to be named.

While those close to the talks said the U.S. scientist had stuck to speaking about the safety of glyphosate -- more commonly known as Roundup -- the implication was clear.

"I think they believe that if they give the scientific evidence on this chemical they will be able to have another kind of debate on whether to use it," said a United Nations official speaking on condition of anonymity.

ADVANTAGES, DISADVANTAGES

U.S. officials argue strongly in favour of aerial spraying. "Aerial eradication is undoubtedly the most effective way," Thomas Schweich, Assistant Secretary of State for International Narcotics and Law Enforcement, told a news conference in Kabul in August.

"You go in, you get large blocks of land in a very short period of time. You do it with minimal loss of life, since you don't have to fight your way in and you don't have to fight your way out. You don't ever negotiate with anybody," he said.

But Britain, Afghanistan's lead international partner on drug eradication and the country with the most troops in the biggest opium-producing area, objects to the practice.

"Our position is very clear. We see more disadvantages than advantages of aerial spraying," said a senior British diplomat. But, he said, "we are prepared to look at a trial of ground-based spraying if the government of Afghanistan wanted one."

Aerial spraying would destroy food crops planted in and around opium, and the Taliban would also use it as propaganda, saying the spray poisons and kills children and livestock and thereby strengthen the insurgency, he said.

The final decision on whether to go ahead with aerial spraying lies with the Afghan government which, for now at least, is standing firmly against it.

"The Afghan government opposes this on two accounts, not only because of the health hazards, but we are also opposing this because we don't want to punish farmers," said Afghan presidential spokesman Humayun Hamidzada.

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