(Adds Sadr statement, paragraphs 8-10)
By Michael Holden
BAGHDAD, March 9 (Reuters) - The U.S. military said on Sunday a recent increase in bombings was not the start of a wider trend in Iraq and violence had decreased overall.
U.S. military spokesman Rear Admiral Greg Smith said he did not think recent security gains were being reversed.
"I would not look at the last few weeks as an increase or a trend, but there has been a sporadic series of events that ... have resulted in significant loss of life," Smith told a news conference.
Smith said the spate of recent attacks needed to be compared with a year ago, when thousands of civilians were dying in sectarian violence between majority Shi'ites and minority Sunni Arabs, with U.S. troops also suffering heavy casualties.
Iraqi police said 68 people died when two bombs exploded within minutes of each other in a popular, crowded shopping area in central Baghdad on Thursday evening, the deadliest single bombing in the capital since last June.
Overall levels of violence are sharply down since last June, when 30,000 extra U.S. soldiers were deployed.
That coincided with a ceasefire by Shi'ite cleric Moqtada al-Sadr's Mehdi Army militia and a decision by mainly Sunni Arab tribal sheiks to turn against Sunni Islamist al Qaeda.
The fiercely anti-U.S. Sadr issued his second statement in three days on Sunday, telling followers they could still defend themselves despite his decision last month to extend a ceasefire for his feared Mehdi Army militia for another six months.
"If a military war is conducted against us by the occupiers we will defend ourselves," Sadr said in a statement bearing his personal seal, his second in three days.
Sadr appeared to be reacting to complaints by members of the militia that rival Shi'ite factions and the U.S. and Iraqi security forces could exploit the ceasefire to attack them.
The number of violent civilian deaths rose sharply in February, the first increase in six months, after bombings which Smith blamed on al Qaeda killed more than 160 people.
Any upsurge in violence could pose headaches for Washington over its plans to withdraw some U.S. troops from Iraq.
Five of 20 combat brigades in Iraq are due to be pulled out by July 31, reducing the overall U.S. military presence from about 160,000 to around 140,000.
On Thursday, the U.S. military said the second of the brigades would be returning from Baghdad, while another based in the city was also earmarked for withdrawal.
"There has been substantial progress -- levels of violence are down by 60-70 percent, civilian deaths and a variety of other categories that we watch are all down," General David Petraeus, the U.S. military commander in Iraq, told Sky News.
U.S. commanders say al Qaeda is the greatest threat to Iraq's security but they are also concerned about "rogue" Shi'ite militias, many which have split from Sadr and which the U.S. military accuses Iran of financing, training and arming.
Tehran has denied the charges, blaming the U.S. presence in Iraq for the violence.
Iranian, U.S. and Iraqi officials held three round of talks in Baghdad last year on ways to end the violence, easing a diplomatic freeze between Washington and Tehran.
But a fourth round of talks has been repeatedly delayed with Washington and Tehran blaming each other for the impasse.
"Direct talks with the United states are not on Iran's agenda but Iran-U.S. talks on Iraq have been delayed due to technical issues," said Iranian Foreign Minister Manouchehr Mottaki on Sunday. (Additional reporting by Luke Baker in London, Edmund Blair in Tehran, Khaled Farhan in Najaf and Waleed Ibrahim and Mohammed Abbas in Baghdad; Editing by Jon Boyle)