Insurgents in Somalia have received huge numbers of weapons in secret shipments from Eritrea, the UN says.
Ugandan African Union troops destroy weapons near Mogadishu
There are now more arms in Somalia than at any time since the civil war started in 1991, the UN report says.
Eritrea, which has repeatedly denied aiding the insurgents, dismissed the report as a "total fabrication".
Meanwhile, three people have been killed in a hand grenade blast at a restaurant in the Somali capital. It is not known who carried out the attack.
It is the first time civilians have been targeted in the current conflict.
In its report to the UN Security Council, the UN Monitoring Group on Somalia said Islamic Courts Union militias - known as the Shabab - had an unknown number of surface-to-air missiles, suicide belts and explosives with timers and detonators.
It said Eritrea had sent at least six SA-18 surface-to-air missiles to the Shabab.
The accusations centre on a chartered Boeing 707 cargo plane that made at least 13 trips from the Eritrean capital, Asmara, to the Somali capital, Mogadishu, sometimes filing false flight plans.
Eritrea denied the flights but the International Civil Aviation Organisation confirmed them, the report said.
Eritrean Information Minister Ali Abdu told Associated Press news agency his country had not provided any assistance to the Shabab.
"It is a total fabrication and the intention of the report is to depict it as if there is a proxy war between Eritrea and Ethiopia," Mr Abdu said.
There are more arms in Somalia than ever before
The presence of government-backed Ethiopian troops in Somalia had only managed to disperse the Islamist fighters and they still posed a serious threat, the report said.
The Islamic Courts Union ruled much of southern Somalia until it was ousted by government-backed Ethiopian troops last year.
Violence has surged since the recent launch of national reconciliation talks and has prompted a fresh exodus of people from Mogadishu. More than 10,000 have fled violence in the past 10 days, the UN says.
An estimated 400,000 people fled the capital during clashes between February and May.
The UN refugee agency says attacks by anti-government elements wound and kill civilians daily.
Somalia has been without a functioning government for 16 years since the start of the civil war.