DUBAI (Reuters) - Iran's top leader rejected possible intervention in Iraq by the United States or any other outside power, accusing Washington on Sunday of trying to manipulate Iraqi sectarian differences to retake control of the country it once occupied.

In remarks published by the official IRNA news agency, Supreme Leader Ayatollah Ali Khamenei added that Iraqis themselves could end violence in their country, where Iran has steadily built up its own influence over the past decade.

"American authorities are trying to portray this as a sectarian war, but what is happening in Iraq is not a war between Shi'ite and Sunnis," said Mr Khamenei, who has the last word in all matters in Shi'te Muslim Iran.

"It is indeed the same old hegemonic order using leftovers of the Saddam (Hussein) regime as its key pieces, and the Takfiri dogmatic elements as foot soldiers," he told judiciary officials, using a term referring to Sunni Islamist militants.

Masked jihadists of the militant Islamic State in Iraq and the Levant (ISIL) have captured swathes of northern Iraq this month, aiming to create an Islamic Caliphate which ignores boundaries set by colonial powers a century ago.

The advance has been driven by an amalgam of Sunni tribal and Islamist militias, and former officers of Saddam Hussein's Baath Party, united in hatred of the Shi'ite-led government, which they accuse of marginalising their sect.

But ISIL has spearheaded the revolt and assaults on cities and towns. Iranian President Hassan Rouhani has said his countrymen will not hesitate to defend Shi'ite shrines in Iraq if need be, but he has also said, like Mr Khamenei, that Iraqis are capable of doing that job themselves.

Thousands of Shi'ite Iraqis have responded to calls to take up arms and defend the country against the insurgency.

Mr Khamenei said he was strongly opposed to intervention by the United States or other countries in Iraq, adding Washington wanted to re-establish control over the oil-exporting country.

"The US is seeking an Iraq under its hegemony and ruled by its stooges," Mr Khamenei was quoted as saying.

Mr Khamenei made no mention of possible cooperation with the US on Iraq, an idea that Mr Rouhani, in answer to a question at a June 14 news conference, said Tehran might consider if Washington tackled "terrorist groups" in the region.

US President Barack Obama on June 19 offered up to 300 Americans to help coordinate the fight against ISIL, but he also urged Prime Minister Nuri al-Maliki, an ally of Iran, to do more to heal the country's sectarian rift.

Mr Obama held off granting a request for air strikes from Mr Maliki's Shi'ite-led government, but earlier this month he ordered an aircraft carrier into the Gulf, readying it in case Washington decides to pursue a military option.

Mr Khamenei said the conflict in Iraq was not sectarian, but was instead between those who wanted Iraq in the US camp and those who sought Iraq's independence, IRNA reported.

The armed campaign "aimed at disrupting Iraq's stability and tranquillity, and threatening its territorial sovereignty ... the row is mainly between those seeking an independent Iraq and those who want Iraq join the US camp".

"America is not pleased with the ongoing (political) process in Iraq, meaning the wide participation in the elections and selecting their own choice (of representatives)," Mr Khamenei said, referring to an Iraqi parliamentary election in April.

Mr Maliki won the largest share of seats in the vote, but must negotiate with other political forces to try to cobble together a coalition to stay in power.

Iran's regional rival Saudi Arabia has repeatedly voiced opposition to any foreign intervention in Iraq, in an apparent message to Tehran.

Saudi Arabia and most other Gulf Arab countries were aghast when the US occupation after Saddam's fall in 2003 brought about elections that empowered Iraq's Shi'ite majority.