US Vice-President Dick Cheney has warned Iran not to interfere in Iraq.
Iran was "fishing in troubled waters", Mr Cheney told US TV
The US government thought it was very important that the Iranians should "keep their folks at home", he said.
His comments come after US forces detained several Iranians in northern Iraq on suspicion of aiding insurgents, accusations rejected by Tehran.
Mr Cheney is the latest member of the Bush administration to warn that the US will take steps against those trying to destabilise the situation in Iraq.
US officials say five Iranian nationals arrested in Irbil on Thursday are linked to the Iranian Revolutionary Guard, accused of training and arming Shia insurgents in Iraq.
Iran's foreign ministry says the men are diplomats and were working at the Iranian liaison office in Irbil. It has demanded their immediate release.
Washington has often accused Iran, or factions within the Iranian government, of aiding Shia groups in Iraq militarily and politically, but has offered little proof of Tehran's alleged activities.
President George W Bush has warned that the US would take a tough stance towards Iran and Syria, which he accused of destabilising Iraq.
But Iraq's Foreign Minister Hoshyar Zebari said on Sunday that Iraq was treading a "thin line", warning that neighbours are beginning to use it as battleground in which to settle scores with the US.
"We fully respect the views, policies and strategy of the United States, which is the strongest ally to Iraq, but the Iraqi government has national interests of its own," Mr Zebari said.
"We can't change the geographical reality that Iran is our neighbour. This is a delicate balance and we are treading a very thin line."
Speaking to Fox News, Mr Cheney said Iran was "fishing in troubled waters" by aiding attacks on US forces and backing Shia militias involved in sectarian violence.
"I think the message that the president sent clearly is that we do not want (Iran) doing what they can to try to destabilise the situation inside Iraq.
He added that the Iranian threat was growing, multi-dimensional and of concern to everybody in the region.
Mr Cheney's television interview formed part of attempts by the Bush administration to promote the new drive to improve security in Iraq, which involves sending an extra 21,500 US troops.
US Defence Secretary Robert Gates is in the UK for a brief visit, meeting Prime Minister Tony Blair and Defence Secretary Des Browne for talks on Afghanistan and Iraq.
US National Security Adviser Stephen Hadley appeared on Sunday morning TV, and Secretary of State Condoleezza Rice has publicly warned Iran and Syria not to destabilise Iraq.
Tehran vehemently denies the charges of interference and says the men detained on Thursday were "involved in consulate affairs".
The outside of the Iranian liaison office raided by US forces
"Their activities were legal and in the framework of the law," Iranian Foreign Ministry spokesman Mohammad Ali Hosseini said in response to the allegations.
The Iranian government has demanded compensation for damage to the building where the men were seized, saying the office and the staff inside should have had diplomatic protection and that the US action was illegal.
"The Americans want to radicalise the atmosphere in Iraq to justify their occupation, but we will act wisely," Mr Hosseini said.
Last month several Iranians were arrested by the US in Baghdad, among them two senior Revolutionary Guard officers. They were released after huge pressure from the Iraqi government.
The Revolutionary Guard, known locally as the Pasdaran, is a parallel military force with its own army, air force and navy.
It was set up to enforce and defend the principles of the 1979 Islamic revolution and answers directly to Supreme Leader Ayatollah Ali Khamenei.