Speaking to Aljazeera's correspondent in Washington, Rumsfeld said holding the talks was an attempt to differentiate between Iraqi fighters and those sneaking in from outside the country.
Rumsfeld said the talks were held in the Iraqi town of Balad; but added that Abu Musab al-Zarqawi or "suicide bombers" were not included in the talks.
The US defence chief also made the same admission in American TV interviews on Sunday after a British newspaper reported that two such meetings had taken place recently at a villa north of Baghdad.
Anti-US commanders "apparently came face to face" with four American officials during meetings on 3 June and 13 June at a villa near Balad, about 40km north of Baghdad, The Sunday Times reported.
When asked on American TV network NBC's Meet the Press about the report of the two meetings, Rumsfeld said: "Oh, I would doubt it. I think there have probably been many more than that."
He insisted the talks did not involve Iraq's most-wanted man, Jordanian-born al-Zarqawi, but were rather facilitating efforts by the Shia-led government to reach out to Sunni Arabs, who are believed to be the driving force behind the anti-US violence.
"We see the government of Iraq is sovereign. They are the ones that are reaching out to the people who are not supporting the government," Rumsfeld said.
"They are not going to try to bring in the people with blood on their hands, for sure; but they are certainly reaching out continuously, and we help to facilitate those from time to time."
However, al-Qaida in Iraq and the Ansar al-Sunna group issued denials on websites that they were negotiating with American or Iraqi officials to end the violence.
"There is a lot talk about false misleading negotiations with the Crusaders and the Jews," the al-Qaida group said in a statement. "We have good faith in our mujahidin brothers that these tricks will not dupe them."
Ansar al-Sunna denied meeting any "crusader or renegade", in its internet statement, saying that jihad was the only way to retrieve the "grace and dignity" of the Muslim nation.
"Our brethren in al-Qaida in Iraq will continue their jihad and fighting of the enemies of God"
Al-Qaida in Iraq statement
The two groups, responsible for most of the deadliest attacks in Iraq since the end of the war, said the reports of negotiations demonstrated that the US military had failed to deal with anti-US violence through military means.
They said their attacks would not end with a US withdrawal from Iraq.
"Our Shaikh Abu Abdullah (Osama bin Laden) has sworn that there will be no safety or security for America," al-Qaida said.
"Our brethren in al-Qaida in Iraq will continue their jihad and fighting of the enemies of God. It is not for a land that we fight but for a religion and a belief."
The Ansar al-Sunna Army said in its statement that even when the Americans were out, their associates in the Iraqi government would remain in Iraq and would remain as targets.
Despite the denials, The Sunday Times report, which quoted unidentified Iraqis whose groups were purportedly involved in the meetings, said the Iraqi groups at the first meeting included the Ansar al-Sunna Army, which claimed responsibility for bombings in Iraq and a Christmas attack that killed 22 people in the dining hall of a US base at Mosul.
Two others were Muhammad's Army and the Islamic Army in Iraq, which, in August, reportedly killed Italian journalist Enzo Baldoni, the newspaper said.
One American at the talks introduced himself as a Pentagon representative and declared himself ready to "find ways of stopping the bloodshed on both sides and to listen to demands and grievances", The Sunday Times said.
"We see the government of Iraq is sovereign. They're the ones that are reaching out to the people who are not supporting the government"
US defence secretary
The official indicated that the results of the talks would be relayed to his superiors in Washington.
Rumsfeld did not provide details about any meetings, saying the "insurgency" had many layers, ranging from disaffected Sunni members of Saddam Hussein's ousted government to "foreign-born terrorists".
"There's no one negotiating with Zarqawi or the people that are out chopping people's heads off," he said.
He also played down the significance of the report.
"I would not make a big deal out of it. Meetings go on frequently with people," Rumsfeld told the US TV network Fox News Channel.
The US officials tried to gather information about the structure, leadership and operations of the groups, which irritated some members, who had been told the talks would consider their main demand - a timetable for the withdrawal of US forces from Iraq, the newspaper said.
The Ansar al-Sunna group has
claimed several deadly attacks
During the 13 June talks, the US officials had demanded that two other anti-US groups, the 1920 Revolution and the Mujahidin Shura Council, cut ties with al-Zarqawi's group, al-Qaida in Iraq, according to the report.
A senior US official said earlier this month that the American authorities had negotiated with key Sunni leaders, who were in turn talking with "insurgents" and trying to persuade them to lay down their arms.
The official, who did not give his name so as not to undercut the new government's authority, did not name the Sunni leaders engaged in the dialogue.
Iraq's former electricity minister, Ayham al-Samarie, meanwhile, said that two insurgent groups - the Islamic Army in Iraq and the Army of Mujahidin - were willing to negotiate with the Iraqi government, possibly opening a new political front in the country.
A senior Shia legislator, Hummam Hammoudi, also said recently that the Iraqi government had opened indirect channels of communication with some Iraqi groups.
The contacts are "becoming more promising and they give us reason to continue", Hammoudi said, without providing details.