The powerful pair met Mr Maliki, a Shiite, and expressed confidence that Iraq will form a national unity government, in an effort to rescue the country from sliding into civil war.
Ms Rice said at a news conference that Mr Maliki "was very focused and very clear that he understood his role and the role of the new government to really demonstrate it is a government of national unity in which all Iraqis could trust."
After intensive discussions with Iraqi legislators, Mr Rumsfeld said he was “most encouraged” that they "have come to reasonable understandings of what the Iraqi people expect of them.”
Shiite Militia
While the American leaders spoke of an optimistic outlook for Iraq, Ms Rice and Mr Rumsfeld issued a note of caution.
They agreed dismantling Shiite militias, which are thought to have carried out executions of the country's former Sunni elite, would take time.
"I think it's too early for detailed discussions about how to go about this," Ms Rice said referring to the militias.
“Iraqis will have to determine how to do this given their circumstances and their capabilities," she said. "How we support them will be part of that discussion."
However, US generals in Iraq have said the new government must move aggressively against the militias.
"That to me is a problem that has got to get fixed," Lieutenant General Peter Chiarelli, the second-ranking US commander in Iraq, told reporters.
"We need a policy on militias, and a policy on weapons. That is something the government is going to tackle early," he said.
Difficult task
The parliament's Shiite bloc nominated Mr Maliki to head Iraq's first four-year post-Saddam Hussein government after a four-month deadlock over the position of the prime minister.
Mr Maliki vowed to rein in both the Shiite groups as well the Sunni insurgents to quell the ongoing sectarian violence in Iraq.
But it will be difficult for the prime minister designate to tackle the Shiite militias as it will force him to confront key political allies like radical cleric Moqtada Sadr. The cleric boasts a thousands-strong private army that has been accused of murdering Sunnis.
Zarqawi tape
The visit by the US leaders came a day after a new video message from Al-Qaeda frontman in Iraq Abu Musab al-Zarqawi was posted on an Internet site.
Ms Rice suggested the message was released out of fear the Iraqis might finally be uniting across the sectarian divide.
"The answer to the Zarqawi video is not anything that the US is saying, it is what the Iraqis are saying, it is having formed a government of national unity despite all the threats and all the violence," she said.
Troop numbers
Meanwhile the defence secretary met General George Casey, the head of US-led forces in Iraq and called on Iraq's new leaders to begin discussions on the future of the US military.
"The question of our forces' levels here will depend on conditions on the ground and discussions with the Iraqi government which will evolve over time," Mr Rumsfeld told reporters.
Gen. Casey said that by the end of the summer, 75 percent of Iraqi brigades will be in the lead in their areas of operations, rising to 80 percent by the end of the year.
He said he was still on his "general timeline" for recommending further reductions in the 132,000-strong US force in Iraq.
The pair's visit came as 14 people were killed in violence across Iraq, including four in a roadside bombing near the restive city of Baquba.
In other developments, Maliki's office issued a statement late Wednesday saying the prime minister designate will now use his full birth name, Nuri al-Maliki, rather than the pseudonym, Jawad, which he took in exile during Saddam Hussein's regime.