Armed men kidnapped 18 African Union personnel in Sudan's restive region of Darfur on Sunday, but later released some of them, the AU says.
The AU said it was worried the troops were being targeted
The identities of those freed and the number still being held were unclear.
The captives included civilian police, military observers, a US envoy and a rebel representative, the AU said.
A spokesman said the kidnappers were believed to be members of a dissident faction of Darfur's rebel Justice and Equality Movement (Jem).
The AU's spokesman in Khartoum, Noureddine Mezni, said the men had been released outside the town of Tina, which is on the border between Sudan and Chad.
The BBC's correspondent in Khartoum, Jonah Fisher, says it is unclear exactly why the men were taken.
But he says harassment and violence against AU peacekeepers is becoming more common.
Two AU peacekeepers and two civilian contractors were killed on Saturday, and another peacekeeper died on Sunday from injuries sustained in the same attack - an ambush the AU blamed on the main Darfur rebel group, the Sudan Liberation Army (SLA).
About 180,000 people have been killed and two million have fled their homes since the Darfur conflict began in early 2003.
"They were taken by a dissident group of Jem," AU Commission spokesman Adam Thiam told Reuters news agency from the Ethiopian capital Addis Ababa.
"We are worried by the two incidents - the killing of the... soldiers and the taking of hostages," Mr Thiam said.
"We are worried because this is targeting the AU as a fighting force, although the AU is there as a peace force."
The deaths were the first among some 6,000 AU troops deployed in Darfur to monitor a truce between pro-government forces and non-Arab rebels.
Splits within the SLA have complicated peace talks as the group's two leaders command loyalties from different fighters and have different views on how to end the conflict, says our correspondent.
Efforts by the African Union to protect their forces better have been hindered by the Khartoum government who have blocked the arrival of armoured personnel carriers.
Last week, the AU accused Sudanese government forces of still carrying out what appeared to be coordinated attacks in Darfur alongside the Janjaweed militia - allegations denied by the Sudanese government.
AU forces arrived in Darfur last year, and from an initial force of 500, the contingent has grown to about 6,200 with financial and logistical support from the European Union, the United States and others.
Peace talks between the Darfur rebels and the government in Khartoum have made little progress while violations have shaken the cease-fire in recent weeks.