A senior U.S. official has warned Iran to stop helping insurgents in Iraq with weapons technology.

Nicholas Burns (file photo)

U.S. Undersecretary of State Nicholas Burns says U.S. forces have detained Iranians suspected of "giving very sophisticated explosive technology to Shi'ite insurgent groups" in Iraq.

The U.S. diplomat made the remark in an interview with National Public Radio, which was recorded for broadcast Thursday.

Describing the situation as very serious, Burns said "the message from the United States is, Iran should cease and desist."

He said that for about two years the United States has been tracking Iranian involvement and has found increasing evidence of Iranain assistance to Shi'ite groups in southern Iraq. He added they now have begun operations "throughout the country," including the Baghdad region.

The Bush administration has repeatedly warned Iran against fueling violence in Iraq, and U.S. forces have detained a number of Iranian officials in recent raids.

In interviews with U.S. media earlier this week, President Bush said the United States will "respond firmly" if Iran escalates its military activities in Iraq. But he also said Washington is not planning to invade Iran.