Iraqi security forces have increased checkpoints and banned motorcycles from Baghdad's streets amid an increase in violence ahead of the withdrawal of US troops from major cities and towns.

Nuri al-Maliki, Iraq's prime minister, on Monday described the June 30 deadline for the US withdrawal as a "turning point" for the country and has declared Tuesday as the country's National Sovereignty Day and a public holiday.

But all police and  army leave has been cancelled amid fears of fresh attacks.

"Our expectation is that maybe some criminals will try to continue their attacks," said Major General Abdul Karim Khalaf, the interior ministry's operations director and spokesman.

"That is why orders came from the highest level of the prime minister that our forces should be 100 per cent on the ground until further notice."

Airport closed

Motorcycles have been banned indefinitely after they were used last week in three separate attacks, killing more than 100 people.

Despite heightened security, a roadside bomb attack on a US convoy in eastern Baghdad wounded six bystanders a day earlier, police said.

In western Baghdad, a car bomb exploded in the parking area of a police academy in Al-Furat district, killing one police officer and wounding seven policemen.

In Sunday's attacks, fighters were believed to have taken advantage of a major sandstorm that reduced visibility to just a few metres in some parts of Baghdad.

The sandstorm forced Baghdad's airport to close and delayed Iraq's first oil bidding process in more than 30 years as international oil companies and representatives could not land in the capital.

The top US commander in Mosul has warned Iraqi army generals that the time has not yet come for his forces to pull out.

"The most dangerous thing that can happen to you and me is that the insurgents separate us, to put a wedge between us," Colonel Gary Volesky said.

The Iraqi army and government have asked that Volesky's troops remain in Mosul past the scheduled withdrawal date.