In the day's bloodiest attack, six Iraqi army soldiers were killed when gunmen ambushed a military checkpoint in a village called Deli Abbas, 40 kilometers (25 miles) north of Baquba, a security source said, adding four insurgents were also killed in the attack.
The ambush was the third attack on a checkpoint in and around Baquba. Two earlier attacks targeted police checkpoints, killing a total of eight people.
In one of them, insurgents approached a checkpoint posing as a wedding party in a convoy of vehicles, one of which was decorated with ribbons and flowers.
Rad Rasheed al-Mulla, the governor of the Diyala province, of which Baquba is the capital, said "the attackers planned to seize control of south, west and southwest entrances to the city."
"But they failed," he said, adding 15 rebels had been killed during the day and 25 others arrested.
Vice president’s sister killed
In Baghdad, gunmen also killed Mayssun Hashemi, sister of newly elected Sunni Vice President Tareq Hashemi, along with her driver. The two were shot dead in an apparent bid to carry out high-profile killings as the country inches closer to form a national unity government.
Mr Hashemi is the head of the Iraqi Islamic Party, a key party in the Sunni-led National Concord Front, a major bloc in the 275-member parliament. The Front holds 44 seats.
In further violence, three Italian soldiers and a Romanian soldier were killed in a roadside bombing in the town of Nasiriyah.
Italy's ANSA news agency quoting Italian intelligence services said that the killings of the four soldiers were claimed by Imam Hussein Brigade, a militant group belonging to Abu Musab al-Zarqawi, Al-Qaeda's frontman in Iraq.
Rice and Rumsfeld leave
The latest bout of violence came as Condoleezza Rice and Donald Rumsfeld ended their surprise visits to Baghdad that were aimed to bolster efforts to form the government.
The high-powered US officials called on Iraqi leaders to usher in democracy which, four months after the landmark December elections for the first full-term post-Saddam Hussein parliament, was still without a government.
"This is Iraq's time and the time for Iraq's newly elected leaders to take on these responsibilities and to represent the desires and the aspirations of the Iraqi people who voted in large numbers, who faced down terrorists in order to vote and express themselves," Ms Rice said before flying out of Baghdad.
Political wrangling has delayed the formation of a new government as the country faces one of its worst bouts of sectarian violence since the 2003 US-led invasion.
Hundreds of Shiites and Sunnis have died in tit-for-tat violence after the bombing of a Shiite shrine in Samarra on February 22.
Last week Iraqi leaders managed to break a deadlock over who would lead the new cabinet by nominating hardline Shiite leader Nuri al-Maliki as prime minister designate. He has to form a new cabinet by May 21.
Rumsfeld said appointing the cabinet, including the sensitive defense and interior portfolios, would "demonstrate to the Iraqi people who went out and voted for them that they have stepped forward, and assumed responsibility for their sovereign nation."
Meanwhile revered Shiite cleric Ayatollah Ali al-Sistani also called for dismantling militias, saying "weapons must be in the hands of government security forces that should not be tied to political parties but to the nation."