"There was an operation on the morning of December 21 based on intelligence. We conducted a raid on a site in Baghdad," he told reporters.
Ten people were arrested and "documents, maps, photographs and videos" seized in the raid, which linked them to "illegal activities", he said. After interrogation it was discovered that two of the 10 were Iranians, he added.
"Debriefing of the detainees and investigation of the seized materials has yielded intelligence linking some of the individuals being detained to weapons shipments to illegal armed groups in Iraq," said a US military statement.
"These 10 individuals are still in the custody of coalition forces," said Caldwell, confirming a pre-dawn raid that has increased tensions between the US-led coalition and Iraq’s embattled government.
Iraqi leaders cited by the New York Times, which broke the story last week, said the Iranians were detained at the compound of powerful Shiite leader Abdul Aziz al-Hakim who met US President Bush at the White House this month.
A spokesman for Hakim denied that the raid took place at his compound, but the site of the arrests marked on a map handed out by the US military was consistent with being in or near the fortified compound.
The home of President Jalal Talabani, who invited the two Iranians to Iraq, is also adjacent to the Hakim compound, in the Karrada neighbourhood.
Talabani has invited the two Iranians to Iraq as part of an agreement between the newly forged allies to improve security, his office said, declaring that the president was "unhappy with the arrests".
US commanders in Iraq regularly accuse Iran of fomenting unrest in its troubled neighbour, but the Shiite-led Baghdad government has insisted on pursuing a policy of closer security ties with Tehran.
If US authorities produce evidence against the detainees it could be the first proof of their longstanding charge that Iranian agents are stirring violence in Iraq by arming and training illegal militias.
Iranian foreign ministry spokesman Mohammad Ali Hosseini was quoted as warning Monday that the arrest of Iranian diplomats by US forces in Iraq "will have unpleasant consequences".
Caldwell confirmed that three more Iranians had been arrested in their vehicle in Baghdad on December 20, one day prior to the raid, but that they had since been handed over to the Iraqi government.
Iraqi officials had already described these three as enjoying diplomatic immunity and that they had been released.
News that two Iranians are in US custody comes amid mounting diplomatic tension between Iran, the United States and the international community, after the UN Security Council voted to impose sanctions on Iran’s nuclear programme.
In response to the vote, Iran defiantly vowed to start work immediately on drastically expanding its capacity to enrich uranium.
Washington accuses Iran of seeking to develop a nuclear weapon, a charge vehemently denied by the oil-rich Islamic republic, which says it only wants to provide atomic energy to a growing population.
Several of the Shiite parties that have risen to power in Iraq since the downfall of former dictator Saddam Hussein have ties to Iran.
Hakim’s Supreme Council for the Islamic Revolution in Iraq (SCIRI) was founded by Tehran to mobilise Iraqi exiles against their own government during the Iran-Iraq war in the 1980s.
Today, the movement is an important part of Iraqi Prime Minister Nuri al-Maliki’s ruling coalition.