Confronted with the violence, which fighters warned would only escalate before Sunday's elections, interim Prime Minister Iyad Allawi said it would be "reckless and dangerous" to fix a timetable for the withdrawal of US troops.
A bearded man, wearing a black and cream shirt, and identified as Roy Hallums, appeared in a videotape broadcast on Aljazeera, begging Arab leaders to secure his release.
The US embassy last month said Roy Hallums, 56, had been captured in November - one of six taken from the office of the Saudi Arabian Trading and Contracting Company in Baghdad.
His video-appeal came amid worsening violence sweeping Iraq before the elections.
At least 15 people were killed on Tuesday, including a senior Baghdad judge and his brother-in-law.
Two civil servants and an Iraqi interpreter working for the US military were also killed, the interior ministry said.
A civilian was killed when he tried to overtake a US convoy in Baghdad, while three police officers, a civilian and two attackers died in clashes southeast of the capital.
In Samarra, north of Baghdad, two Iraqi soldiers and a civilian were killed during a clash with fighters, police said.
Loyalists of al-Qaida ally Abu Musab al-Zarqawi also claimed to have attacked more than a dozen polling centres overnight.
Iraqi police said the attacks caused severe damage to centres across Saddam Hussein's home province of Salah al-Din in northern Iraq, but no casualties were reported.
"Trained snipers will be ready to kill the apostates who go to the electoral lairs," warned a statement signed by al-Zarqawi's group.
Another armed group, the Islamic Army in Iraq, in a statement ordered "its forces everywhere to escalate attacks to the maximum".
Confronted with the rising tide of violence, Allawi said he would not fix a timetable for the withdrawal of US troops.
"I will not set final dates because dates now would be both reckless and dangerous," he said.
US soldiers killed
Northeast of Baghdad, a US Bradley fighting vehicle rolled into a canal during a patrol, killing five soldiers from the army's 1st Infantry Division and injuring two others, the military said on Tuesday.
The accident, which is under investigation, occurred near the town of Khan Bani Saad during fierce sandstorms on Monday night.
Another US soldier died of wounds from a roadside bomb that hit an American patrol in Baghdad, the military said.