The second-ranking U.S. commander in Iraq said Wednesday that the number of rocket and mortar attacks in Iraq that can be linked to Iranian-sponsored fighters has fallen in recent weeks.

Lt. Gen. Lloyd Austin attributed the decline mainly to efforts by Iraqi forces to choke off radical elements of Shiite militias in the southern cities of Basra and Amarah.

Austin said he did not know whether the drop-off in attacks is an intentional gesture by Iran. Austin spoke at his headquarters at the main U.S. military compound just west of Baghdad.

Meanwhile, bombs killed 13 people, including four Iraqi policemen, in Iraq on Wednesday as military officials released data showing attacks have dropped sharply over the past year.

Violence in Iraq persists despite security gains that have been attributed to a range of factors, including the 2007 U.S. troop surge, a Sunni revolt against Al Qaeda in Iraq and government crackdowns against Sunni extremists and Shiite militias.

In Washington, the U.S. general who led efforts to train Iraq's army and police units said Wednesday that progress is mixed and long-term U.S. help is needed.

A homicide car bomber targeted a military convoy carrying a senior Iraqi commander in the northern city of Mosul, the Iraqi military said.

Eight civilians were killed, and 41 people were injured, including seven of the commander's guards, the military said. Lt. Gen. Riyadh Jalal Tawfiq, chief of operations in Ninevah province, escaped unharmed.

Brig. Gen. Khalid Abdul-Sattar, an Iraqi military spokesman, described the attack as an attempt to assassinate the commander.

Also Wednesday, a bomb went off outside a bank in the one-time Sunni insurgent stronghold of Fallujah west of Baghdad, killing four policemen and a civilian, Iraqi police said.

An initial blast went off at the scene at 6:30 a.m., drawing a crowd and police to the area. Then a second bomb exploded, causing the deaths and wounding 15 people, a police official said. The injured included an Iraqi television cameraman.

The police official spoke on condition of anonymity because he was not authorized to talk to the media.

A U.S. soldier was killed and two were wounded in an attack around noon Wednesday on an American convoy in Samarra, north of Baghdad, the U.S. military said.

The bombing of a revered Shiite shrine in Samarra in 2006 and 2007 fueled fierce sectarian fighting between Sunnis and Shiites in many parts of Iraq, though security in the city has improved since then.

The attacks on Wednesday came a day after Iraqi officials stepped up pressure on the United States to agree to a specific timeline to withdraw American forces, a sign of the government's growing confidence as violence falls.

The Iraqi military said Wednesday that the number of "terrorist attacks" in June declined 85 percent from the same period a year ago.

An average of 25 attacks took place each day in June, compared with 160 during the same month in 2007, said Iraqi army spokesman Maj. Gen. Qassim al-Mousawi during a news conference. He did not provide details on the individual attacks included in the figures.

In Washington, U.S. Army Lt. Gen. James Dubik said Iraq's security forces have grown from 444,000 to 566,000 since he assumed command of the Multi-National Security Transition Command in June 2007 and are better able to execute operations on their own.

But the fast-growing force still lacks experienced leaders and the ability to train all its new recruits, Dubik told the House Armed Services Committee.

"As I often said to my command in Baghdad, 'Progress doesn't result in no problems, it results in new problems,"' he said in his written testimony.

Assailants killed a policeman Wednesday in a drive-by shooting in Mosul, where many insurgents are believed to have relocated after facing intense military pressure in Baghdad and other urban centers.

Police in Ramadi, west of Baghdad, said workers who were rebuilding a primary school discovered 22 bodies, most of them under concrete in a playing field. A police official, who earlier said only five bodies were discovered, said the remains were believed to have been buried more than a year ago.

Relatives of missing people were summoned to the site Tuesday. Those identified included a Muslim cleric whose wife recognized his clothes, the official said on condition of anonymity because of security reasons.

Ramadi, like Fallujah, was a stronghold of the Sunni-led insurgency, but violence there has dropped since Sunni tribal leaders and their fighters formed an alliance with the U.S. military.