The bodies of 23 people have been found following the kidnapping of bus drivers north-east of Baghdad, police say.
Mr Rumsfeld thanked US troops for their service in the war
Earlier, a suicide bomber killed seven and wounded 20 people after walking into a restaurant in eastern Baghdad.
The latest violence comes as US Defence Secretary Donald Rumsfeld made a surprise visit to Baghdad for meetings with US commanders and Iraq officials.
Mr Rumsfeld said talks with PM Nouri Maliki would focus on security and a reconciliation process for Iraqis.
They would also discuss training for Iraqi security forces but not the question of US troop withdrawals, he said.
Also on Wednesday, Prime Minister Maliki told the Iraqi parliament that a plan by militants to occupy parts of western Baghdad had been thwarted.
"There was a plan to occupy Baghdad's districts west of the Tigris but Iraqi forces were able to thwart this occupation," he told parliament.
Baghdad has been hit by a wave of sectarian violence that has left dozens dead, despite a recent tightening of security in the capital.
Gunmen seized about two dozen people from the bus station in the town of Muqdadiya, in Diyala province, some 90km (60 miles) north-east of Baghdad, earlier on Wednesday.
Reports say some if not all who were taken were bus drivers.
Mr Rumsfeld will discuss the ongoing violence with PM Maliki
Police in the town told the BBC that 23 bodies were later found.
An Iraqi army commander in the area, Maj Gen Ahmed al-Awad, told Iraqi state TV that all the victims were Shias. This has not been officially confirmed.
Continuing violence will be high on the agenda during Mr Rumsfeld's meeting with Prime Minister Maliki.
On his flight to Baghdad - following a surprise visit to Afghanistan - Mr Rumsfeld said security in Iraq depended as much on the process of political reconciliation between Sunni and Shia factions and the strengthening of government ministries as on military success against insurgents:
"Success in those areas will determine the success from a security standpoint."
But he stressed there would be no talk about plans to reduce US troops levels in Iraq, saying the Iraqi government had to complete a "comprehensive review" of security needs before it would be possible to make those decisions.
"I don't talk deadlines," he said, when asked for a timescale.
'Persistent and ruthless'
There is growing domestic pressure to bring home some of the 129,000 US troops in the country, amid increasing anti-war sentiment among the US public in a year when congressional elections are due.
Following his arrival at an air base north of Baghdad, Mr Rumsfeld gave a speech to US troops and thanked them for their service in a difficult war.
He told the soldiers that a definition of "what victory means" involves "first and foremost... helping the Iraqi people take the fight to the enemy", the Associated Press reports.
He described the enemy in Iraq as "persistent and ruthless" and warned they "are not going to quit".
The defence secretary said he was not planning to raise the subject of investigations into alleged abuses against Iraqi civilians by US forces.
He said he expected no change in the agreement which gives US soldiers immunity from Iraqi domestic law, although this has been questioned by Mr Maliki.