The United Nations is investigating the use of its vehicles by suicide bombers who killed 17 African Union peacekeepers at their main base in Somalia, a senior official said on Saturday.
President Sheikh Sharif Ahmed's government said on Friday that Islamist rebels had seized more UN vehicles in readiness for suicide attacks.
"There are very large numbers of UN vehicles in Somalia that have been used for a variety of projects," Mark Bowden, the UN humanitarian coordinator for Somalia told Reuters.
Bowden said the UN had been given the chassis number of one of the vehicles used in the attack. "We are trying to trace whether it's a UN vehicle," he said.
Bowden said the attack on the peacekeepers' base next to Mogadishu airport on Thursday would not weaken the UN's resolve to deliver aid to half the Somali population but it could hamper operations on the ground.
"We have to take greater precautions around Mogadishu, clearly the airport is more at risk and that will affect our ability to move staff and humanitarian goods," he said.
Insurgents overran UN compounds in Jowhar and Baidoa in May and July, seizing aid supplies and vehicles. This has given rise to speculation that the vehicles used in the suicide attacks were captured then.
Bowden said the vehicles could have come from elsewhere. "Depending on the marking, they could have been vehicles that have been brought in from the Eritrean peacekeeping operation or they are vehicles that have been used on projects over the years," he said.
The al Shabaab rebel group, which Washington says is al Qaeda's proxy in Somalia, controls much of the south and parts of the capital Mogadishu.
Together with Hizbul Islam, the group has been fighting government troops and African Union peacekeepers to impose its own strict version of sharia law throughout Somalia.
Al Shabaab ordered traders at the country's biggest market, Bakara, to join their fight or vacate their stalls, businessmen said. The group also told them to contribute financially and in kind to their cause.
More than 18,000 Somalis have been killed since the start of 2007 and driven another 1.5 million from their homes.
Bowden said severe drought for the fifth year in a row had compounded the effects of rising violence and pushed half of the population into food aid dependence.