Prime Minister Tony Blair says there is "no excuse" for the bloodshed to continue in Iraq with the formation of a new government.
Mr Blair says the new Iraqi government is a big step forward
Mr Blair said this was a "new beginning" which would allow Iraqis to "take charge of their own destiny".
He was speaking at a news conference with Iraqi Prime Minister Nouri Maliki after landing in Baghdad on Monday.
Meanwhile violence continued across the country with at least 11 people killed in shootings and bomb attacks.
One of the victims was the director general of the youth and sports ministry, Hamid Hassan, who was shot dead on his way to work in southern Baghdad.
In other violence:
- Four police are killed in a bomb attack on their patrol vehicle 50km (31 miles) south of Baghdad
- At least three die in a bomb attack on a Baghdad market
- Two die in a car bomb attack on a police patrol in the city's Zafaraniyah district
- A mobile phone company employee is shot dead in Baqouba. AFP news agency reports four more deaths in separate attacks in the city
- Gunmen kill a police colonel and wound another officer in Samarra, AP reports.
'Three years of struggle'
Mr Blair's arrival in Baghdad from Kuwait was shrouded in secrecy because of the security situation.
The new government was sworn in on Saturday, but key ministries remain unfilled.
Mr Maliki vowed to use all means necessary, including "maximum force", to restore order.
Mr Blair said it had taken "three years of struggle" to reach the formation of a government.
"For the first time, we have a government of national unity that crosses all boundaries and divides, that is there for a four-year term and [was] directly elected by the votes of millions of Iraqi people."
He said there was "no vestige of an excuse" for the "terrorism and bloodshed" to continue.
"If the worry of people is the presence of the multinational forces, it is the violence that keeps us here. It is the peace that allows us to go."
In a second news conference after talks with Iraqi President Jalal Talabani, Mr Blair said he wanted Iraqis to be in control of every aspect of their own nation.
"That's why we were discussing that progressively provinces can be yielded up to Iraqi control and this is something that's done on a timetable with conditions set down by your government, so we will work to that - that's important," PA news agency quoted him as saying.
Mr Maliki said the plan was for Iraqi forces to take over control of operations from the multinational forces as they become ready, province by province, beginning in June.
He said reconstruction was a priority, and that talks were continuing to appoint the ministers of defence and the interior.
This is the second time the British prime minister has visited Baghdad's heavily-fortified Green Zone, and he is the first world leader to visit the new government.
The attacks and killings have continued apace
His show of support for the new national unity government indicates how important he considers it for Iraq's future - and perhaps too his own legacy, says the BBC's political correspondent James Landale, who is travelling with the prime minister.
Our correspondent says the prime minister seems genuinely hopeful that the establishment of an elected, full-term government marks a new start for Iraq.
But Mr Blair still concedes the new government may not succeed. His call to insurgents to lay down their arms, says our correspondent, depends on whether the new government can draw them into the political process - something that is far from guaranteed.
A senior British official travelling with the prime minister said the withdrawal of the present multinational force should be accomplished within four years.