President Yoweri Museveni has accused Kenyan revenue officials of being to blame for a month-long shortage of diesel fuel in Uganda.
Museveni flew into Juba amid tight security
The halting of fuel deliveries amounts to a blockade and is causing a "very big problem", says Mr Museveni.
Diesel comes to Uganda from Kenya via a pipeline but technical problems since March have caused low fuel supplies.
Uganda has been negotiating with Kenya to allow the fuel to be transported in trucks from the refinery at the coast.
"If you stop fuel from coming to Uganda, that is a blockade. This is serious and it can create a very big problem," Mr Museveni told crowds in Jinja during the Labour day celebrations.
Kenya's Revenue Authority has licensed five oil companies to ferry the fuel in trucks but there is disagreement over a demand that they pay a refundable deposit.
Kenyan officials argue that this deposit will ensure the companies do not divert the fuel to the Kenyan market, but Uganda's authorities say it amounts to double taxation.
Like other countries in the region, Uganda suffers from frequent electricity blackouts, and many people use diesel to power private generators.
Uganda has hydro-electric plants but these are not enough as yet. Last week, the World Bank approved a huge loan to fund a new dam project.