Pace said there was less information available on the Iranian weapons transfers to Afghanistan than on Iranian weaponry and support to Iraq.
"We do not know [in Afghanistan] with the same clarity we know in Iraq who is delivering those weapons, who is involved," Pace said.
|The US military said that Iranian weapons were found in the city of Kandahar|
Before the Sunni Taliban were driven from power, Iran supported numerous armed Afghan opposition groups against them, including the Northern Alliance which played a role in toppling the Taliban after the 2001 US-led invasion.
The US government and military has repeatedly accused Iranian forces of supplying and training Iraqi insurgents.
Iranian officials have dismissed the US accusations.
Meanwhile in Afghanistan, Taliban fighters cut off a road northeast of the Afghan capital Kabul on Tuesday, prompting government forces to launch a counter-attack, a provincial governor said.
Taliban fighters attacked police posts and a government headquarters in the Tagab district of Kapisa province, 70 km from Kabul.
"The Taliban have cut off the road. Both sides are aligning their forces for more fighting," Abdul Sattar Murad, Kapisa's provincial governor, told Reuters news agency.
Murad said up to 300 Taliban fighters were involved in the attacks, about 50km southeast of the main US military base at Bagram.
A US military spokesman said Afghan forces had been involved in fighting in the area but said he had no information about US involvement.
There have been renewed attacks by Taliban fighters over recent weeks, after they vowed to launch a spring offensive backed by thousands of suicide bombers.
Nato and US-led forces have swept areas in the south over recent months to thwart the threatened offensive.
Taliban fighters also captured a government headquarters in Qarabagh district in Ghazni province, to the southwest of Kabul, on Tuesday night, travellers from the province said.