MARJAH, Afghanistan -- Armed with assault rifles and fistfuls of American dollars, government agents drove deep into Afghanistan's biggest poppy-growing region Wednesday to begin enforcing a plan to eradicate the opium-bearing crop.

As soldiers with Kalashnikov rifles and grenade launchers looked on, tractors chewed up fields of poppy in one part of Helmand province, which produces most of Afghanistan's opium. Farmers said they had little choice but to accept state compensation money.

"They have gunmen, they have cars, they have force," said Durjan, a 23-year-old farmer who planned to plant beans where poppies once stood.

"We have no option."

At the urging of the United Nations and foreign governments, the weak Afghan government is rushing to wipe out the crop that provides the raw material for heroin just two weeks before most farmers harvest the plant.

Afghanistan was once the source of 70 percent of the world's opium. The Taliban successfully banned poppies in 2000, but farmers quickly planted them again as the U.S. bombing campaign helped push the Islamic militia from power late last year.

The government initially offered poppy farmers $250 to destroy a jirib, an Afghan land measure equivalent to half an acre, but farmers in Helmand said the compensation did not cover their cultivation expenses and staged violent protests.

On Sunday, security forces shot and killed eight farmers who were protesting the state poppy policy in the Helmand district of Kajaki.

The government has since raised the amount of compensation to $350 per jirib, said Shabaz Ahmedzai, an adviser to interim Prime Minister Hamid Karzai.

The amount is closer to the $400 farmers say it costs them to plant a jirib, but still far less than the $1,700 they could expect to receive if they harvested the poppies.

Durjan said he expected to be paid $1,750 by the government for his five jiribs.