Richard Boucher, assistant U.S. secretary of state for South and Central Asia, told reporters that Washington would like Iran to continue its previous "generally positive role" in reconstruction in neighboring Afghanistan.
However, over the past year, "there have been increasing concerns raised over Iran's behavior in Afghanistan - involvement that goes beyond the sort of cultural, commercial and educational, and starts going into reports of involvement in political areas or reports of contacts and arms supply to the Taliban," he said. "These are things we are watching very carefully."
U.S. military officials raised worries of a wider Iranian role in Afghanistan on Tuesday when Gen. Peter Pace, chairman of the Joint Chiefs of Staff, said in Washington that U.S. forces had intercepted Iranian-made mortars and plastic explosives intended for Taliban fighters in Afghanistan.
Counterterrorism officials in Washington have said a handful of senior al-Qaida operatives who fled to Iran after the war in Afghanistan in 2001 may have developed a working relationship with a secretive military unit linked to Iran's religious hard-liners.
U.S. officials caution, however, that any Iranian link to fighting in Afghanistan, notably in providing weapons to Taliban fighters, remains cloudy.
Iran has rejected the charges.
Boucher, who was in Brussels for talks with European Union officials on Afghanistan and the wider central Asian region, reiterated appeals for European allies to contribute troops to bolster NATO forces in the south and east of Afghanistan, which is facing the heaviest Taliban resistance.
Spain, Italy, Germany and France, members of both the EU and NATO, have refused to send more troops or to move existing forces in Afghanistan.
Boucher said the aim of this year's reconstruction efforts was to ensure the spread of government authority from Kabul to outlying regions by building roads, setting up schools and police stations, and providing electricity.
"It's to build the Afghan government and its ability to provide security, provide economic opportunity, provide good government for the people of Afghanistan," Boucher said.
The United States has around 27,000 troops in Afghanistan and has offered US$11.6 billion (EUR8.56 billion) in new aid for the country.
The 27-nation EU in February proposed a EUR600 million (US$813 million) package for Afghanistan for health, justice and rural development over the next four years, and EU nations has agreed to set up a police training mission that could be deployed as early as May.