KABUL - A bomb concealed on a bicycle killed 13 people Wednesday in southern Afghanistan, as the Pentagon's top military officer said NATO forces hope to reverse the Taliban's momentum in the south with an upcoming offensive in Kandahar.

Forty-five people, including eight children, were wounded in the blast, which occurred in the Nahr-e-Sarraj district just north of Lashkar Gah, the capital of Helmand province, deputy provincial police chief Kamaluddin, who uses one name, said.

The bomb exploded near a crowd gathered to receive free vegetable seeds provided by the British government as part of a program to encourage them not to plant opium poppy, provincial government spokesman Daoud Ahmadi said.

Casualty figures fluctuated several times during the day because of communications problems in the area, Kamaluddin said.

No one claimed responsibility for the attack, which President Hamid Karzai blamed on "enemies of the Afghan people who are against peace."

The acting provincial head of agriculture, Ghulam Sahki, said the blast could have been the work of drug dealers trying to stop the alternative crop program. NATO and the Afghan government hope poppy farmers in the south, where most of the world's opium is grown, will adopt legal crops with the help of cash incentives and programs like the seed distribution. The narcotics trade helps fund the Taliban insurgency.

A recent NATO operation in the Helmand town of Marjah, south of Lashkar Gah, struck at the heart of the Taliban opium business. NATO, U.S. and Afghan forces took control of the town in a three-week offensive in February and early March but face a fearful and mistrustful population as they work to set up a functioning government.

In Kabul, Adm. Mike Mullen, chairman of the U.S. Joint Chiefs of Staff, said Wednesday the operation in Marjah was moving forward successfully and that an upcoming offensive in and around the main southern city of Kandahar would be key to stopping the Taliban's growing influence in the south.

Kandahar remains the spiritual heartland of the insurgency. Mullen told reporters it would be military's main focus going into the summer when the operation there kicks into high gear.

"It is a cornerstone in reversing the momentum for the Taliban," Mullen said.

He said that about half of the 30,000 new troops promised by President Barack Obama have yet to arrive and said most of those will be headed to Kandahar city and the surrounding province.

As in Marjah, the Kandahar offensive will focus on winning over the population and installing government as quickly as possible, he said.

But Mullen stressed that Kandahar will be more difficult to take and hold because of the size of the city -- about a half million in the urban area and another half million in the area around it -- and the entrenched Taliban presence.

"Kandahar is not Marjah, we understand that. It is a much bigger challenge and in that regard has much greater potential to achieve this goal of reversing the momentum," Mullen said.

Seizing control of Kandahar would help to put the Afghan government in a position of strength to pursue reconciliation talks with insurgents, Mullen said.

But, he said, those talks would be premature right now.

"This must be done from a position of strength," Mullen said. "I don't think we're in that position of strength right now."

Mullen said that a number of power brokers and foreign governments wield influence in the Kandahar area. The admiral said he was briefed Tuesday about "a significant shipment of weapons" for insurgents from Iran into Kandahar "not too long ago." He declined to provide further details.

Also Wednesday, Karzai visited the Salang Pass where an avalanche killed at least 171 people in February. The 3,800-metre-high Salang Pass is the major route through the Hindu Kush mountains that connects the Afghan capital of Kabul to the north of the country. He discussed with officials the need to build retaining walls to prevent snow slides onto the road and to construct shelters for stranded travellers.