The US still has about 133,000 troops in Iraq
The Iraqi authorities are tightening security in preparation for next Tuesday's deadline for US soldiers to pull out of the country's cities.
All police leave has been cancelled and extra troops have been drafted in, amid a spate of bomb attacks this week which has left 250 people dead.
Prime Minister Nouri Maliki says the attacks may be aimed at stirring up sectarian tensions.
But he says he is confident his government can safeguard security.
"[The attackers] want to sweep delight from the Iraqi people's hearts. They have revealed their real intentions," said Mr Maliki.
"But this will not bend our determination and will for what we have agreed upon - that is, to return security responsibilities to our military and police forces."
US forces have already left many bases in cities such as Baghdad.
But our correspondent says the troops are not far away, in positions just outside urban areas where they are poised to intervene should they be called on to help by the Iraqi forces.
Most of the deadly bombings this week have targeted Shia areas - leading the Iraqi authorities to blame Sunni militants from the al-Qaeda group.
The authorities have beefed up security across urban areas - with special attention given to controlling access to markets.
Militants have frequently targeted markets, hoping to inflict as many casualties as possible.
Under an agreement with the Iraqi authorities, most of the 133,000 US troops in Iraq are due to leave the country's cities and towns and withdraw to military bases by 30 June.
Combat operations across Iraq are due to end by September 2010 and all US troops will be out of the country by the end of 2011.