Several hundred troops have been airlifted by the UN to a remote corner of the Democratic Republic of Congo to deal with Ugandan rebels on its soil.
DR Congo's military had been put on full alert
The presence of Lord's Resistance Army (LRA) rebels on Congolese territory has triggered a diplomatic row with Uganda.
Last week, Uganda threatened to send its soldiers into DR Congo after them.
But DR Congo has warned that such a move would pose a threat to international peace and security and called for the UN to impose sanctions.
In 1998, Uganda was one of several countries to become embroiled in DR Congo's five-year civil war in which an estimated three million people were killed.
The LRA, who abduct children in fight for them, has been waging a war with no clear agenda for the past 19 years.
The rebels normally operate in northern Uganda and southern Sudan, but a group has recently moved into north-east DR Congo.
"We have transported 300 Congolese soldiers to Aba in our helicopters and another 200 are on the way there by road," United Nations military spokesman Thierry Provendier said, Reuters reports.
The LRA is notorious for child abductions
The force will number 1,000 men by the end of this week, he said.
A deadline set by the Congolese government for foreign militias to leave the country expired on Friday.
Meanwhile, the BBC's Will Ross in Kampala says trucks full of Ugandan soldiers and military hardware have been moved to the Congolese border.
The Ugandan army says this is a precautionary measure to prevent LRA rebels in DR Congo from attacking Ugandan civilians.
But Mulegwa Zinhindula, a security adviser to the Congolese president, told the BBC's Focus on Africa programme that DR Congo's military had been put on full alert.
Last week, Uganda's President Yoweri Museveni said if the Congolese authorities failed to disarm LRA members, the Ugandan army would enter DR Congo to do so.
"Uganda has no right to say it will invade DR Congo when there are existing mechanisms between us for resolving respective problems," Mr Zinhindula said.
"Congo will never allow Ugandan troops to cross inside Congolese territory," he said.
In a letter to the UN Security Council on Monday, DR Congo's UN ambassador Atoki Ileka asked for sanctions to be imposed on Uganda including an arms embargo and the suspension of international aid.
Mr Zinhindula denied DR Congo was attempting to inflame the situation by referring the matter to the Security Council.
It is unclear how many LRA rebels are in DR Congo, but Mr Zinhindula said quoted figures of 400 were exaggerated by the Ugandans.