The US ambassador to Iraq, Zalmay Khalilzad, has said US officials have held talks with some groups linked to the Sunni-led Iraqi insurgency.
Mr Khalilzad told the BBC that he believed the talks had had an impact, as the number of attacks on US troops by Iraqi militants had fallen.
But he ruled out negotiating with those he called Saddamists or terrorists.
He also urged Iraq's politicians to break the deadlock over who should lead the new government of national unity.
Iraq's President Jalal Talabani has said the dispute, four months after elections, is one of the main obstacles to political progress in the country. The current prime minister, Ibrahim Jaafari, has rejected growing pressure to resign, saying Iraqis must be allowed to choose their leader democratically.
Kurdish and Sunni parties, and many within Mr Jaafari's dominant Shia alliance, want him to withdraw.
"The national unity government needs the approval of all groups for the prime minister, otherwise how can he lead a national unity government?" Mr Talabani said in a BBC interview.
Mr Khalilzad told the BBC's Baghdad correspondent that it was important for Iraq to have a good, competent and unifying government but Iraqi politicians should agree its composition as quickly as possible.
"The patience of the Iraqi people and the international community is running out," he said.
Mr Jaafari is so far resisting calls to stand aside
Mr Khalilzad admitted that Iraq was going through a difficult transition and that civil war remained a real risk.
He acknowledged that the US was talking to some groups with ties to the insurgency.
"We are talking to people who are willing to accept this new Iraq, to lay down their arms, to co-operate in the fight against terrorists," he said.
He said he believed the contacts were producing results.
"The number of attacks on the coalition is down. I think last month was the second lowest month of attacks against the coalition."
Mr Khalilzad would not specify which groups the US had had contact with but said they would not talk to people he called Saddamists or terrorists seeking a war on civilisation.
That is usually a reference to al-Qaeda figures such as the Jordanian militant, Abu Mussab al-Zarqawi.
However, the ambassador said militia groups, which he described as the infrastructure of civil war, were just as much of a problem.
Mr Khalilzad is seen as one of the architects of US President George W Bush's decision to overthrow Saddam Hussein three years ago.
He has been more prepared than most US officials to admit things have not worked out as planned.
He said the risk of sectarian war breaking out in Iraq remains if ethnic divisions between its Kurdish and majority Arab population developed, and warned it could turn into a wider regional conflict.
Iraqi officials have warned people in Baghdad not to gather in crowds near mosques or markets.
The interior ministry said it had received intelligence that several car bombings were being planned in the capital and possibly in some southern provinces, the Associated Press reported.
On Thursday, a car bomb killed at least 10 people and injured about 30 others in Najaf in southern Iraq, the first serious bomb attack in the holy city of Najaf in many months.