Donald Rumsfeld's arrival on Monday came as a spokeswoman for the US-led forces said that at least 30 Taliban fighters had been killed in an early morning air strike in the Helmand province.
The US defence secretary said that drug money was fuelling the Taliban's rise, and called on Europe and Russia to do more to help stem the demand for narcotics from Afghanistan's poppy fields.
Lieutenant General Karl Eikenberry, commander of the 23,000 US troops in Afghanistan, told reporters travelling with Rumsfeld that the Taliban had extended their influence from traditional strongholds into areas where the government was weak.
Rumsfeld said Eikenberry was consulting with Karzai on US troop levels required in the region, but the general declined to comment on whether more troops will be needed.
The United States had hoped to cut its force levels in Afghanistan from 19,000 to 16,000 this year, but instead are adding more troops as the Taliban mounted its largest spring offensive since 2001.
"I wouldn't want to comment on troop levels," Eikenberry said. "This is going to require adjustments, we're going to look at what capabilities are needed beetween Nato and ourselves. We'll figure that out."
The Nato-led ISAF is in the process of taking over responsibility for southern Afghanistan from a US-led force. The ISAF is then expected to move into eastern Afghanistan late this year.
The US will continue to be a major contributor of combat forces, helicopters and logistics to ISAF and maintain a separate US-commanded counter-terrorism strike force, Eikenberry said.
He said the transition would bring about 3,500 British troops into Helmand province, where, until recently, the main outside presence has been an 80-person provincial reconstruction team and a 50-member special forces unit.