Darfur has been torn by conflict since 2003 when mainly non-Arab rebels took up arms against the Arab-led government, accusing Khartoum of politically and economically marginalising the region.
Violence is down from its peak in 2003 and 2004, but a surge has forced more than 130,000 people to flee their homes since the start of the year, according to the United Nations.
The joint African Union-United Nations peacekeeping mission (UNAMID) said it was escorting three buses carrying the Darfuris when it was stopped by a "large unidentified armed group in military uniforms and seven jeep-mounted guns" on Sunday.
UNAMID said it had conflicting reports about whether the displaced people had been released. No official government sources had confirmed any release, they said.
The conference for displaced people and refugees was scheduled to be held on March 25 and 26 in Nyala, the capital of South Darfur.
Khartoum refuses to recognise the court, which it says is biased against leaders who refuse to kowtow to Western powers.
In 2008, the United Nations said about 300,000 people may have died in Darfur's war, a figure some activists say is too low. The government has put the death toll at about 10,000.