Gates' visit follows close on the heels of John Negroponte, the US deputy secretary of state, and Admiral William Fallon, the US commander in the Middle East, who visited Iraq with similar objectives.
On the ground, the repercussions of Wednesday's al-Askariya shrine bombing continued to be felt.
The largest Sunni mosque in the Basra region in southern Iraq was blown up and destroyed early on Friday morning, a senior security officer said.
"Let the people of Iraq vote if they want the US to stay or leave"
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But there were no casualties in the attack on the lha mosque in al-Zubair, a town around 15km southwest of Basra city, in keeping with the pattern since Wednesday of reprisals against property, not people.
According to General Ali al-Mussawi, a top Basra security official, a group of men posing as TV cameramen went to the mosque on Friday morning and asked guards if they could film inside the shrine.
They instead planted bombs inside the structure and demolished it, he said. There were no reported casualties.
The guards were detained afterward for questioning, al-Mussawi said.
Lieutenant-Colonel Christopher Garver, the US military spokesman, attributed the low level of retaliatory attacks to a three-day curfew in Baghdad that ends on Saturday and the fact that the Iraqi police and army were doing a better job.
It could also be a sign that the presence of more US forces on the streets of Baghdad was already helping, he said.
Earlier on Friday, the US military has said it had completed the deployment of extra 28,000 troops to Iraq as part of the "surge" aimed at curbing sectarian violence.
Garver said all US troop reinforcements had arrived in Iraq, but it could take months to fully feel their weight.
|The Sunni mosque in al-Zubair which was|
destroyed on Friday in a bomb attack [AFP]
The reinforcements are part of the security push launched in February aimed at re-establishing security.
It will take 30 to 60 days for the new arrivals, who have taken total US troop levels in Iraq to 160,000, to win the confidence of residents and start getting the intelligence needed to counter violence, Garver said.
That means troops might not be operating at full capacity until August.
General David Petraeus, the top US military commander in Iraq, and Ryan Crocker, the US ambassador, are due to report on the success of the security build-up in September.
More US losses
Friday provided no respite from bad news for the US military either. An air force F-16 fighter jet crashed during a close air support mission for ground forces.
Separately, the US military said five soldiers had died.
Three of the soldiers died when a bomb exploded near their vehicle on Thursday during operations in Kirkuk province, in northern Iraq.
Another soldier was wounded in the blast.
A fourth soldier was killed by small-arms fire the same day in Diyala province, northeast of Baghdad. And another soldier died on Wednesday in a non-combat related incident.
In other news, an Iranian foreign ministry spokesman said on Friday that three Iranian diplomats were detained and interrogated by US forces in Iraq.
Mohammad Ali Hosseini, the spokesman, condemned the detentions, which he said occurred while the diplomats were travelling from Iraq to Iran on Friday to spend the weekend back home.
Ali al-Dabbagh, the Iraqi government spokesman, confirmed most of the details in the Iranian foreign ministry account, saying the diplomats were headed to the Iranian border when they were intercepted by an Iraqi police patrol in southeast Baghdad.
He said they were briefly questioned and allowed to continue, but they were later detained by a US military patrol.
He said the Americans later released the three men.