The attack came at the end of a bloody day that saw the death of a US-backed Iraqi Governing Council leader, a bombing at a Baghdad hotel and more attacks on US soldiers.
Police Lieutenant Abbas Khodeir said the mortar hit about 9.10pm (1710 GMT) in the town nearly 70km from the capital, but could not say who fired it.
Officials of Baquba's general hospital said they treated seven wounded in the blast, including a 12-year-old boy.
Six other wounded people were taken to the Diyala hospital.
Eight soldiers were wounded, three seriously, when their convoy came under attack in the northern city of Mosul.
A Somali security guard was killed at a Baghdad hotel when a bomb, left on the pavement beside it, shattered windows and sent debris flying.
The hotel housed journalists from US television network NBC. The network said it would continue covering events in Iraq.
In more resistance activity late Thursday US military authorities announced that one of their soldiers had died in a rocket-propelled grenade attack in another northern city, Kirkuk.
"This is not an evacuation (of UN staff), just a further downsizing, and the security situation in the country remains under constant review"
With no sign of a let-up in the fighting, the United Nations said it was withdrawing 19 of its 105 international staff in Iraq due to concerns over security.
UN offices in Baghdad have twice come under attack.
UN spokesman Fred Eckhard called the shift of the staffers to nearby Amman, Jordan, "a temporary redeployment of international staff in Iraq".
He said 42 international staff remained for now in Baghdad and 44 in northern Iraq, down from 105, and "these numbers can be expected to shrink further over the next few days."
"This is not an evacuation, just a further downsizing, and the security situation in the country remains under constant review," Eckhard told reporters.
UN Secretary-General Kofi Annan has agonised over staff security since a 19 August bomb attack on UN Baghdad headquarters killed 22 people, including head of mission Sergio Vieira de Mello.
A second bombing on the compound on Monday killed an Iraqi policemen and wounded 19 others including local UN workers.
Before the 19 August bombing, about 600 international staff worked in Iraq, helped by around 4000 Iraqi employees.
UN's vital role - Washington
The White House said it still wanted the United Nations to play a vital role in Iraq despite the staff withdrawals.
"We certainly understand their concerns and understand their reasons for pursuing a reduction, but they have a vital role to play and we want them to continue to play that vital role," White House spokesman Scott McClellan said.
US Secretary of State Colin Powell claims the views of the UN Security Council are converging
Thursday's attacks occurred ahead of a report expected to lay open US President George Bush to further criticism over his main justification for launching a pre-emptive war without the UN's blessing and over the objections of traditional allies, France and Germany, and of Russia.
As Washington reaches out to the United Nations for help to find countries willing to join its efforts to stabilise and rebuild Iraq, a senior American official said the eagerly awaited US report was expected to say there was no proof Iraq had chemical or biological weapons.
Such a report would provide powerful ammunition for the rising number of critics to attack Bush and his most forthright ally, British Prime Minister Tony Blair, over their decision to invade Iraq on the premise that former Iraqi President Saddam Hussein's weapons of mass destruction posed an imminent threat.
US forces have been searching unsuccessfully for chemical, biological and nuclear weapons in Iraq for more than five months.
A senior official said Washington hoped Iraq's former defence minister, who was given effective immunity from prosecution when he surrendered to US forces last week, may be able to help track down the weapons.
"I am pleased and I think my colleagues in the P5 are pleased that we're seeing some convergence of views with respect to the resolution"
US Secretary of State
US Secretary of State Colin Powell said Thursday that there was "some convergence of views" among the members of the UN Security Council on a new resolution for Iraq.
"I am pleased and I think my colleagues in the P5 are pleased that we're seeing some convergence of views with respect to the resolution," Powell said after the foreign ministers of the permanent five Security Council members met over a lunch at the United Nations headquarters.
Transatlantic split still open
France and Germany want a swifter handover of power to Iraqis as a condition for supporting Washington's efforts. The United States says it would be rash to hurry the process.
In his address to the UN General Assembly, Russian President Vladimir Putin avoided the Iraqi dispute, focusing on the need for tougher action to fight acts of “terrorism”, whether they are in Baghdad or Russia's rebel Chechnya.
Some Governing Council members have also pressed for a quick return to Iraqi self-rule. Members of Iraq's delegation at the United Nations denied any rift with the United States, but said they hoped a new constitution could be ready by May, paving the way for democratic elections and self-government.