Earlier, Parliament Speaker Hajim al-Hassani, a Sunni, said Shia and Kurds had made amendments to address Sunni concerns about federalism and purging former ruling party members.
But Sunni negotiator Fakhri al-Qaisi said his side saw no "essential changes" in that offer.
He said Sunnis would not accept the draft described by Shia and Kurds on Friday as complete. Sunni leaders have urged voters to reject the charter in a 15 October referendum if it does not meet their demands.
Federalism or decentralisation
Al-Hassani had said the constitution would be submitted to parliament on Sunday. The legislature, overwhelmingly Shia and Kurdish, may vote on it or simply refer it to voters.
The speaker said Shia and Kurds proposed to delay consideration of federalism's details until later and recognised that many members of Saddam Hussein's Baath Party were not criminals.
Al-Qaisi said Sunni delegates met al-Hassani to present new charter wording.
The constitution has been
rejected by some Iraqis
"We are waiting for an answer," al-Qaisi said. On federalism, he said the Sunnis wanted "decentralised" provinces with a "special case" for Kurdish areas. He said the Shia position on barring former Baathists from public life "is totally rejected".
He said the Sunnis would confer later with US Ambassador Zalmay Khalilzad. If the charter clears parliament without Sunni blessing, it would be a blow to the Bush administration, which has insisted that Sunni participation is critical to producing the constitution.
Shia leaders and their Kurdish allies are gambling that the draft constitution will win approval in the referendum.
But if two-thirds of voters in any three of Iraq's 18 provinces reject the constitution, it will be defeated, and Sunnis form a majority in at least four provinces.
Sunni clerics have urged them to vote "no" if the draft does not serve Sunni interests.
In an effort to mollify Sunni Arabs, the US military announced on Saturday that nearly 1000 security detainees had been let out of Abu Ghraib prison the past several days.
The move, the largest release to date, came after Sunni negotiators appealed to the government to free thousands of prisoners - most of them Sunnis - who have been languishing in jail for months without being charged.
Written versions of the Shia-Kurdish concessions, which were presented to Sunni leaders on Friday, were not released.
Al-Hassani said the concessions involved delaying setting the details of how to implement federalism - or the establishment of self-ruled regions - until a new parliament is elected in December, presumably with more Sunni members than the current one.
Shia and Kurds proposed to delay
a discussion of federalism
Many Sunni voters did not participate in the 30 January elections, and the parliament has few Sunni members.
He said the concessions also recognized that "not every person who joined the Baath Party is a criminal" and needed to be barred from public life.
Occupiers and collaborators
Opponents of the constitution within the Sunni and Shia communities condemned the draft.
An alliance of rejectionists, including the Sunnis' Association of Muslim Scholars and the movement of Shia cleric Muqtada al-Sadr, condemned a "political process which had been led by occupiers and their collaborators".
"We consider this draft as a next step of this process which does not represent the peoples' will," the group declared.
"We have major suspicions about the honesty of the next referendum, which will take place under occupation and with neither international nor Arabic and Islamic supervision."
Shia negotiator Ali al-Adib said his group offered major concessions on federalism and the programme to purge former Baath members from government and public life.
"Regarding the powers given to provinces, this is the right of the Iraqi people and we can give up this right," al-Adib said.
"It could be regulated by the next National Assembly, this article is optional ... as for the Baath issue, there were crimes and there should be punishment for the criminals. This is a right of Iraqis that we cannot give up."
The split pointed to fundamental differences on visions for the new Iraq, including whether it continues as a centralized state or becomes a federation based on religion and ethnicity.
Sunnis fear that federalism, demanded by the Shia and Kurds, not only would establish a giant Shia state in the south but also encourage Kurds to try to expand their self-rule region into northern oil-producing areas.
That would leave the Sunnis cut off from Iraq's oil wealth.
Sunnis had insisted the issues of federalism and the fate of Baath party members be deferred to the next parliament, in which they hope to have more members. Sunni Arabs form an estimated 20% of the 27 million Iraqis but won only 17 of the 275 parliament seats because many Sunnis boycotted the 30 January election.
Sunnis resent attempts to ban former Baath Party members from government posts or political life because they believe that would deprive them of livelihood and prevent the country from using the talents of thousands of professors, senior executives and others who joined the organization to advance their careers.
However, Shia and Kurds suffered under Saddam, and hatred for the Baath Party runs deep.