"Our forces are on our own soil and, based on the known international borders, this well belongs to Iran," the armed forces command said in a statement, quoted by Iran's Arabic-language Al-Alam satellite television.
On Friday, Iraq's state-owned South Oil Co in the southeastern city of Amara said "an Iranian force arrived at the field … It took control of Well 4 and raised the Iranian flag even though the well lies inside Iraqi territory."
Baghdad has demanded that "Tehran pull back the armed men who occupied well No 4" and condemned the incident as "a violation of Iraqi sovereignty."
It was the first serious incident between the two neighbours since the US-led invasion of 2003 that toppled Saddam Hussein, whose forces fought a 1980-1988 war against Iran.
Many leaders of Shiite parties who were exiled to Iran during the Saddam era are now in power in Baghdad.
"It's a sovereignty issue" which has to be resolved by Iraqi leaders, the chairman of the US Joint Chiefs of Staff, Admiral Michael Mullen, told reporters during a visit to Baghdad.
But Iran's foreign ministry's spokesman Ramin Mehmanparast accused "external sources" of working to damage relations between Tehran and Baghdad, the official IRNA news agency reported.
And a senior Iranian MP also tried to play down the dispute.
"The claim that Iran has occupied an Iraqi oil well is strongly rejected," Alaeddin Borujerdi, head of parliament's national security and foreign policy commission, told IRNA.
The issue was "being examined through diplomatic channels," he said, blaming "foreign media for such propaganda."
In Baghdad, Deputy Foreign Minister Mohammad al-Hajj Hamud told AFP that an Iranian unit made up of around a dozen soldiers and technicians was still posted at the disputed well on Saturday.
"We summoned Iran's ambassador to Baghdad yesterday (Friday) to tell him that this attack is unacceptable and our ambassador to Tehran delivered a note to their foreign ministry to ask them to pull out their troops," he said.
Hamud said it was the first time Well 4 had been taken over. "In the past, the Iranians would try to prevent our technicians from working on the well … by firing in their direction," he said, adding Iraq had dug the well in 1974.
The Iraqi official said the incident came a month before a joint commission starts work on demarcating the two countries' land and sea border along the Shatt al-Arab waterway in the south.
Well 4 is in the Fauqa Field, part of a cluster of oilfields which Iraq unsuccessfully put up for auction to oil majors in June. The field has estimated reserves of 1.55 million barrels.
Iraqi government spokesman Ali al-Dabbagh, meanwhile, said 11 Iranian soldiers were involved and that Baghdad was demanding the removal of the Iranian flag.
In southern Iraq, a US military spokesman told AFP that the incident at Well 4 was non-violent but the latest in a series of such activity along the frontier.
"The oilfield is in disputed territory in between Iranian and Iraqi border forts," said the officer at Contingency Operating Base Adder, just outside the city of Nasiriyah.
The well lies about 500 metres (yards) from an Iranian border fort and about one kilometre from an Iraqi border fort, US Colonel Peter Newell said.
But it falls on the Iraqi side of a border agreed between the two countries, according to the US officer, who added that there were five other fields in disputed territory.
World oil prices rose on Friday, with markets edgy over the dispute.
New York's main futures contract, light sweet crude for January, rose 71 cents to close at 73.36 dollars a barrel, while in London, Brent North Sea crude for February delivery settled 38 cents higher at 73.75 dollars a barrel.
