UN monitors have found evidence that arms shipments to the Somali government have been diverted to clan militias and in one case were destined for a Shebab rebel commander.
A confidential report by the Somalia and Eritrea Monitoring Group, seen by AFP on Friday, found "high level and systematic abuses in weapons management and distribution" by Somali authorities.
In February 2013 the United Nations Security Council voted to partially lift an arms embargo against Somalia, seeking to help the beleaguered government in its battle against Islamist guerrillas.
But the council imposed restrictions requiring notification of shipments, banned certain heavy weapons and mandated the Monitoring Group to watch how matters proceeded.
On February 6, the coordinator of the UN monitors wrote to the chairman of the Security Council committee overseeing the sanctions to present a report that raised serious concerns.
The report, which is not binding on UN members, recommended a reversal of the loosening of the embargo, to try and stop arms shipped to the Somali government falling into the wrong hands.
"However, an alternative recommendation to the committee would be to introduce, at the minimum, enhanced notification and reporting requirements, if not a partial tightening," it said.
Somalia's government struggles to control its own territory, despite the support of a large African Union force, and the Shebab rebels regularly launch devastating attacks despite recent defeats.
Since the arms embargo was made more supple last year, Somalia's allies Ethiopia and Djibouti have sent several large shipments of mainly infantry weapons to government forces.
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In addition, according to the US experts' report, a senior Somali government minister bought weapons from "a foreign government in the Gulf" and shipped them without notifying sanctions monitors.
This in itself constituted a breach of the embargo, the report said, adding: "Indeed, after delivery, some of the weapons were moved to private locations in Mogadishu. Investigations are ongoing."
The UN monitors reported that they were able to obtain photographs showing rifles that were shipped for use by the Somali army on open sale in Mogadishu markets, with their serial numbers ground off.
Senior Somali officials have also, the report alleges, been implicated in the transfer of weapons meant for the national army to militias tied to the Abgaal and Habar Gedir clans.
Somalia's President Hassan Sheikh Mohamud is a member of the Abgaal clan, and the report says the team found evidence tying a network of officials to arms shipments to the group's forces.
The group has "obtained documentary evidence corroborating information that a key advisory to the president, from his Abgaal sub-clan, has been involved in planning weapons deliveries to Shebab leader Sheikh Yusuf Isse 'Kabukatukade'."
The Security Council is expected to review the arms embargo in March, and could decide either to lift restrictions entirely, maintain the monitored sanctions regime or tighten it.
Since the fall of dictator Siad Barre in 1991, Somalia has been prey to a series of conflicts between clan, religious and criminal militias.
Mohamud's election in September 2012 has given rise to hope that the country may once again achieve stable government, and he has received strong support from the West and regional allies.