The total number of South Sudanese facing famine could rise to 5.5 million in July if nothing is done to address the food crisis, the experts said.
WEAPONS POUR IN
The report was released ahead of a special Security Council meeting on South Sudan on Thursday that will be chaired by British Foreign Secretary Boris Johnson.
The meeting could once again revive calls for an arms embargo, which was rejected despite warnings from the United Nations of a risk of genocide in South Sudan.
While the previous US administration pushed for a ban on weapons sales, President Donald Trump's government has yet to make clear its stance on ending one of Africa's worst conflicts.
Borders with Sudan and Uganda continue to be key entry points for weapons supplies to South Sudanese forces and some shipments are also entering from the Democratic Republic of Congo, according to the report.
The panel cited information from high-ranking South Sudanese military and intelligence officers that Egypt had shipped military equipment, small arms, ammunition and armoured vehicles to South Sudan over the past year.
Experts are investigating the delivery this year of two L39 jets from Ukraine that were sold to Uganda, but may have ended up in South Sudan, as well as a contract with a Seychelles-based company for a very large quantity of armaments.
In comparison, opposition forces have received limited supplies of light weapons ammunition, the report said.
After gaining independence from Sudan in 2011, South Sudan descended into war in December 2013, leaving tens of thousands dead and 3.5 million people displaced.
The United Nations is pushing regional leaders to exert pressure on Juba to end the violence that has turned tribal, pitting Kiir's Dinka community against ethnic Nuer, Shilluk and other groups.