"We have no way of knowing whether this was targeted at the United Nations. This is a dangerous situation. Only the restoration of law and order can put an end to these attacks," he said.
The UN World Food Program was targeted in a July 6 grenade attack in Mosul, and four days later, the agency issued a release citing concern over the security situation in Iraq.
UN special representative Sergio Vieira de Mello left Iraq on Sunday. He is to report to the UN Security council on Tuesday.
Meanwhile, in Najaf, thousands of followers of Shiite Muslim cleric Muqtada al-Sadr set out from the Imam Ali shrine on a 9.5km march to US headquarters there, shouting slogans against the new, US-sanctioned Iraqi Governing Council and the Americans.
"Long live al-Sadr. America and the Council are infidels," chanted the crowds. "Muqtada, go ahead. We are your soldiers of liberation."
US troops prevented the demonstrators from entering the headquarters and soldiers barricaded the building with Humvees. The crowd dispersed after clerics read out an appeal by al-Sadr to go home.
Earlier, al-Sadr said in a statement read inside the shrine that he wanted coalition forces to leave Najaf and allow Iraqis to handle security for themselves. In his Friday sermon, the cleric said he was recruiting a private army but fell short of calling for armed struggle against the US occupation.
A coalition official dismissed the threat, but said no private armies will be tolerated.
"The only army in Iraq will be the new Iraqi army, which is being formed," the official said, speaking on condition of anonymity. "Anyone found with an unlicensed weapon will be arrested."
The US-led coalition has begun building what it hopes will eventually be a 40,000-strong military force.
On the first day of recruiting Saturday, the coalition processed 5,000 applications at centers in Mosul, Baghdad and the southern city of Basra, the official said.
Al-Sadr, thought to be 30 years old, is not considered a high-ranking Shiite cleric.
Most of his support is by virtue of his being the son of Imam Mohammed Sadiq al-Sadr, a top Shiite religious leader assassinated by Saddam's agents in 1999.
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