A series of attacks across Iraq killed more than a dozen people, including three U.S. soldiers, authorities said Thursday. The killings came despite assurances from U.S. officials that progress was being made to improve security in the capital.

At least 13 Iraqis, including civilians, soldiers and police, were killed in bombings and shootings, becoming the latest casualties in the country's ongoing sectarian and political violence.

One soldier was killed on Thursday afternoon in the capital when his patrol was attacked by gunmen, the U.S. military command said in a statement.

Another American soldier died Thursday when his vehicle was hit by a roadside bomb south of Baghdad, the military said. The identities of both soldiers were withheld pending notification of their families.

And another soldier was killed south of Baghdad on Wednesday during a raid to capture "foreign terrorists," the U.S. command said, adding that "two terrorists were also killed during the firefight."

"The unit was conducting operations to rid Iraq of foreign terrorists known to be operating in the area," the command said in a statement, giving no further details.

CountryWatch: Iraq

Meanwhile, two car bombs in separate areas of Baghdad killed four civilians and wounded nine people, including two policemen, police said. A bomb concealed by trash wounded four day laborers.

Northeast of the capital, a bomb in a minivan killed three policemen and wounded a driver in the city of Baqouba, while a roadside bomb on the city's outskirts killed three Iraqi soldiers, authorities said. Shootings in Baghdad and Mosul left three people dead.

Political and sectarian violence across Iraq spiked after the Feb. 22 bombing of a Shiite mosque in the town of Samarra, north of Baghdad. The bloodshed claimed 3,500 lives last month, making July the deadliest month since the U.S.-led invasion in March 2003 and stoking fears of a civil war.

Sunni Arab insurgents have been regularly attacking U.S. and Iraqi troops, mostly in the Baghdad area and in the province of Anbar to its west.

But coalition authorities have been saying that a new security operation to tackle violence in the capital is producing results. About 12,000 additional U.S. and Iraqi troops have been brought in to Baghdad as part of the security effort.

"I think that there's been great progress in the security front here recently in Baghdad," said Gen. John Abizaid, the top U.S. commander in the Middle East, after meeting with Iraqi President Jalal Talabani.

Abizaid said he and Army Gen. George Casey, the top U.S. commander in Iraq, were "very optimistic that the situation will stabilize."

He said that comments he had made previous regarding the possibility of Iraq sliding into civil war had been misinterpreted.

"I believe there is a danger of civil war in Iraq, but only a danger. I think Iraq's far from it," Abizaid said.

Referring to the violence in Baghdad, Casey said: "I think everybody has seen an improvement in the situation in Baghdad over the last weeks because of the operations of the Iraqi security forces supported by the American Army."

In a column in the Wall Street Journal published Wednesday, U.S. Ambassador to Iraq Zalmay Khalilzad wrote that "the Battle of Baghdad will determine the future of Iraq, which will itself go a long way to determining the future of the world's most vital region."

Elsewhere in Iraq, police arrested the suspected local leader of an insurgent group in Mosul, 225 miles northwest of Baghdad, police Brig. Saaid al-Jubouri said.

Authorities identified the suspect as Abdul Rahman Ali Abdul Rahman, also known as Abu Hajir, believed to be the local leader of the Mujahedeen Army.

The group has claimed responsibility for scores of attacks, including the April 2005 downing of a helicopter carrying 11 civilians, including six Americans. It was among 11 Sunni insurgent groups that offered in June to halt all attacks if the United States agrees to withdraw foreign forces from Iraq in two years.

Meanwhile, U.S. and Iraqi authorities released 55 detainees in Baghdad as part of Prime Minister Nouri al-Maliki's reconciliation efforts. The prisoners, who were released in the city's central bus station, chanted slogans supporting pro-Shiite cleric Muqtada al-Sadr.

Authorities did not give any details on what crimes they had been detained for, but one freed prisoner, Dhahir Abu Muhanid, told AP television he had been accused of detonating a bomb near coalition troops.

"I spent one year and nine months in Bucca prison. I'm innocent," he said.

Elsewhere in Iraq, police raided a house on the outskirts of Nasiriyah, about 200 miles southeast of Baghdad, arresting two alleged kidnappers and freeing an Iraqi civilian abducted four days ago, police said.

Abductions are widespread, with many carried out for ransom. Last week, a Catholic priest was kidnapped in Baghdad, and Pope Benedict XVI appealed for his release.

To the east, in the city of Kut, 100 miles southeast of the capital, police on Thursday found four handcuffed bodies dumped separately in the streets. All had been shot, said Mahmoud Khazim of the Kut city morgue.

Police in Mosul said they found the bodies of a five-member family — two women, one man and two young men — in a house in the east of the city.

The latest deaths raised to at least 2,615 the number of U.S. military personnel who have died since the beginning of the Iraq war in March 2003, according to an Associated Press count.

The figure includes seven military civilians. At least 2,078 died as a result of hostile action, according to the military's numbers.