The official, who spoke on condition of anonymity, said the closures took effect on Wednesday.

"The plan to close the borders went into effect last night. Many points were closed, but I can't confirm that all were shut," Lieutenant-Colonel Christopher Garver told Reuters earlier.

Iraq had said it would shut the borders for 72 hours. The US military said on Wednesday the aim was to allow procedures and the layout of border positions to be revamped.

Washington accuses Syria of allowing foreign fighters to cross its long, porous borders into Iraq and says Iran is supplying weapons, including roadside bombs, to Iraqi militants to kill US soldiers. The two countries deny the claims.

Iraq said on Tuesday it would close four border crossings with Iran and two with Syria to try to quell sectarian violence in Baghdad.

A large-scale US and Iraqi security operation began to bite yesterday, with thousands of troops on the streets and fighter jets screaming a high-decibel message in the skies overhead.

Iraqi officials said that corpses of only five murder victims had been found overnight -- extremely low by the Iraqi capital's recent bloody standards -- in what they said was a sign that death squads had gone into hiding.

Commanders have warned that it will take months for "Operation Law and Order" to make a lasting difference, and that US troop levels will not peak until May at the earliest, but the plan was clearly moving into gear.

"Intelligence-focused searches accompanied by clearing operations were conducted by coalition and Iraqi security forces in multiple locations across Baghdad," said US army spokesman Lieutenant Colonel Scott Bleichwehl.

Meanwhile, a suicide car bomber attacked an Iraqi police station in the western city of Ramadi, killing 11 people including four police, the US military said Thursday.

Wednesday's attack also wounded 21 people, a spokesman said.

Iraqi medical and security sources in the city, a hotbed of Sunni insurgents, said the police station was severely damaged in the blast.

They said the local police chief was among those killed, but that was not confirmed in the US statement.

Iraqi security forces are a constant target for insurgent fighters trying to undercut the Shia-dominated unity government of Prime Minister Nuri al-Maliki. (Reuters, AFP)