The Democratic Republic of Congo has accused Ugandan troops of illegally crossing onto its territory - charges denied by the Ugandan army.
The Ugandan army denies its troops have crossed the border
DR Congo said one Ugandan soldier had been killed after clashing with Congolese soldiers in pursuit of Ugandan Lord's Resistance Army rebels.
Earlier this month, Uganda asked for permission to pursue the notorious LRA rebels in DR Congo.
Uganda withdrew its troops from DR Congo in 2003 after a five-year war.
Last year, the World Court ordered Uganda to pay compensation for looting DR Congo's resources during the conflict, which involved several African armies.
Uganda and Rwanda tried to topple the DR Congo government, which they accused of backing rebel groups.
A statement from DR Congo's foreign ministry said that a company (about 120 men) of Ugandan soldiers had crossed the border in vehicles equipped with canons.
"A note of protest has been handed to the embassy of Uganda in Kinshasa," a statement said.
Ugandan army spokesman Maj Felix Kulaigye denied the Congolese accusations.
"It is not true that our troops entered DRC," he said.
DR Congo is due to hold its first multi-party elections in 40 years later this year - to elect a government to rebuild the country after years of conflict and mismanagement.
Parts of the east remain volatile and armed groups from Burundi, Rwanda and Uganda, as well as locals, continue to roam, killing, raping and looting.
The LRA has been waging a war with no clear agenda against Uganda's government for 20 years.
Their fighters have been active in northern Uganda, southern Sudan and more recently eastern DR Congo.
The conflict has gained notoriety for the LRA's massacres and its tactic of kidnapping children for use as soldiers and sex slaves.