Jose Graziano da Silva said the FAO is powerless to even curb the stampede that began several years ago and declared that a "sheriff" is needed to restore order before the phenomenon gets totally out of control.
At a time when food shortages are becoming more common as the world's population keeps swelling, da Silva told Britain's Guardian newspaper: "I don't see that it's possible to stop it.
"They're private investors. We don't have the tools and the instruments to stop big companies buying land. Land acquisitions are a reality.
"We can't wish them away but we have to find a proper way of limiting them. It appears to be like the 'Wild West' and we need a sheriff and law in place."
The land grabs were triggered by the 2007-08 world food price crisis, with countries such as Saudi Arabia and South Korea buying or leasing vast tracts of land in Africa, Asia and Latin America.
They planned to grow food there that would be exported to their own populations, none of it going to the people of famine-plagued Africa.
Many of the land agreements in African states, where corrupt governments are the norm, involve under-the-table deals with high-ranking officials who enrich themselves at the expense of peasants who've farmed the lands for generations and were displaced with little or no compensation.
In a December 2011 report, the International Land Coalition, a global network of civil society and farmers' organizations, estimated the stampede for land claimed 494 million acres in sub-Saharan Africa in 2000-10.
The lack of secure land tenure has facilitated "an astonishing buying spree across Africa," Jeffrey Hatcher, director of global programs with Rights and Resources Initiative, observed recently.
The non-governmental organization, which has headquarters in Washington, seeks to promote land and policy reforms in 20 African countries.
Despite the efforts of FAO and non-governmental organizations to set up mechanisms to control the stampede, Olivier De Schutter, the U.N. special rapporteur on the right to food, says governments in sub-Saharan Africa or southeast Asia with poor governance, or tainted by corruption, will continue to attract big-spending investors whatever the cost.
"The international community should accept it has a role in monitoring whether the rights of land users ... are effectively respected," he said.
The British charity Oxfam says much of the land taken over is being used by speculators, including U.S. banks, hedge funds and other high-profile institutions, to grow biofuels to sell for hefty profits rather than produce food, either for poor Africans or hungry Arabs.
GRAIN, an international non-profit organization that works to support small farmers and community-controlled food systems, observed in a June report that many land grabs are about water sources that go with the land.
It said African land grabs are in effect "large-scale industrial agricultural operations" that require last amounts of water for irrigation, and most are found within the continent's river basins, the Nile in particular.
"Hidden behind the current scramble for land is a worldwide struggle for control over water," the report noted.
"Those who have been buying up vast stretches of farmland in recent years, whether they are based in Addis Ababa, Dubai or London, understand that the access to water they gain, often included for free and without restriction, may well be worth more over the long term than the land deals."
There are concerns the land-grabbing across Africa and other developing regions could trigger a series of conflicts if governments fail to protect the rights of their people, according to recent studies.
"Controversial land acquisitions were key a factor triggering the civil wars in Sudan, Liberia and Sierra Leone, and there is every reason to be concerned that conditions are ripe for new conflicts to occur in many other places," RRI's Hatcher cautioned.
In an April report, RRI estimated 500 million people in sub-Saharan Africa depend on 3.46 billion acres of communally held farmland that's been a primary target of governments and investors seeking to produce food specifically for non-African populations.