A UN report has warned that "systematic abuses" by Somalia's government have allowed weapons to be diverted to warlords and al-Shabab militants.
The report for the UN's sanctions committee calls for the restoration of an arms embargo on Somalia that was relaxed last year.
It said a key adviser to Somalia's president has been involved in planning weapons deliveries to militants.
Somalia's mission to the UN has questioned the report's validity.
The confidential 14-page report was compiled by the UN's Somalia and Eritrea Monitoring Group, a panel of independent experts supervising compliance with the sanctions regime.
It said the group had "identified a number of issues and concerns over current management of weapons and ammunition stockpiles" by Somalia's government.
These, it said, "point to high-level and systematic abuses in weapons and ammunition management and distribution".
The Security Council imposed the embargo on Somalia in 1992 as the country descended into two decades of unrest.
It was hoped that last year's easing of sanctions on light weapons such as assault rifles and rocket-propelled grenades would enable an internationally-backed government to better arm its security forces against al-Shabab.
Al-Qaeda-aligned Al-Shabab claimed responsibility for a car bomb attack on Thursday near the international airport in the Somali capital, Mogadishu. At least six people were killed.
The new report outlines the difficulties faced by the monitors in gaining access to weapons stockpiles in Somalia and obtaining information about its growing arsenal.
It said shipments of weapons from Uganda and Djibouti could not be accounted for.
Many weapons were being diverted away from security forces and into the hands of militias loyal to powerful clans or for sale in private arms markets in Mogadishu, it added.
Within these clans, one key adviser to the president has been involved in planning weapons deliveries to al-Shabab, the report said.
"Given the gaps in information... it is impossible to quantify what the scale of diversion of weapons stocks has been," it said.
"However, the Monitoring Group has obtained other pieces of qualitative evidence that point towards systematic abuses by the (Somali army)."
It recommend the restoration of the full arms embargo, or at least stricter rules.
The Somali mission to the UN said the report contained "very serious allegations" which will "cause damage to the legitimacy of the government."