Nepal's human rights commission has said that Maoist rebels have kidnapped 12 security personnel in the eastern hill district of Bhojpur.
The commission has urged the rebels to free the hostages who it says were kidnapped after clashes on Wednesday.
Casualties were feared on both sides, but the authorities said they had not received any details.
Violence between the rebels and security forces has escalated in recent month.
More than 12,000 people have been killed in the 10-year Maoist insurgency in Nepal.
Show of strength
The rebels say they want to replace the monarchy with a communist republic.
The Maoists have not commented on the allegations of taking hostages after the clashes in Bhojpur.
The fresh clash occurred less than a week after a rebel attack on the eastern hill district of Khotang.
At least five policemen were killed in the attack and 60 prisoners managed to escape after rebels broke into a jail, authorities said.
None of the prisoners were said to be Maoist cadres.
Correspondents say the attack appeared to have been intended as a show of strength by the rebels.
The attack came hours after the rebels said cadres had been ordered not to carry out "physical attacks on unarmed people".
King Gyanendra assumed direct control of Nepal on 1 February, dismissing parliament and accusing politicians of failing to tackle the Maoists.