The US defence secretary has warned against political purges when a new Iraqi government takes power.
Mr Rumsfeld praised progress in the formation of a new leadership
On a surprise visit to Iraq, Donald Rumsfeld said Baghdad must not allow goals of achieving democracy and defeating the insurgency to be blocked.
The US is worried that officers from the security forces who served under Saddam Hussein may be dismissed, the BBC's Baghdad correspondent says.
Mr Rumsfeld's trip follows the election of a new president and prime minister.
The visit comes as the UN special envoy to Iraq, Ashraf Qazi, called for greater attention to be paid to human rights in Iraq.
He told the UN Security Council that more attention needed to be paid to due process for detainees.
And, in developments on the ground:
- At least five Iraqis are killed and three others injured in a car bomb attack on a US convoy in the northern city of Mosul, just ahead of Mr Rumsfeld's visit to the city
- Insurgents in western Baghdad ambush a convoy carrying a senior interior ministry official, killing a bodyguard and injuring three others, ministry officials say
- Dozens of insurgents are rounded up in a major raid by US and Iraqi forces in central and southern Baghdad on Monday morning.
Mr Rumsfeld urged the new Prime Minister, Ibrahim Jaafari, and the new President, Jalal Talabani, to avoid delays in setting up the new administration.
"It's important that the new government be attentive to the competence of the people in the ministries and that they avoid unnecessary turbulence," Mr Rumsfeld told journalists on his pre-dawn flight into Baghdad.
He said it was important for the security forces to continue building their strength because US forces were not going to be there for ever.
The defence secretary also praised progress made in forming a leadership that spanned Iraq's diverse groups.
Mr Jaafari acknowledged there would be problems ahead but expressed optimism that the new government could meet those challenges.
Mr Talabani said he hoped a new cabinet would be formed by the end of the week.
After the invasion, the US administrators disbanded Iraq's former army but the decision is now widely acknowledged as a mistake and the interim government has re-employed many experienced soldiers who served under Saddam Hussein.
The BBC's Caroline Hawley in Baghdad says Washington is now concerned that purges of these officers brought back to the security forces could undermine efforts to defeat the insurgency.
Iraqi Shias suffered brutally under the rule of Saddam Hussein and may seek retribution, she says.
A special force of 10,000 commandos, set up last year, is almost entirely drawn from the former military. One official told the BBC that some of them had participated in crushing a Shia uprising in 1991.
Mr Jaafari, a Shia, was appointed on Thursday shortly after Kurdish leader Mr Talabani was sworn in as interim president.
The transitional government's main task will be to oversee the drafting of a permanent Iraqi constitution and to pave the way for elections in December.
Mr Rumsfeld is flying to Mosul later for a meeting with Kurdish leader Masoud Barzani.