Yemeni forces have launched an operation to free two Chinese oil workers kidnapped on Sunday.
The two Chinese men and two Yemeni colleagues were taken by tribesmen in the eastern Shabwa region.
Yemen's powerful tribes frequently kidnap people in order to bargain with the central government in disputes.
Yemen, the Arabian peninsula's poorest country, is currently battling a resurgent al-Qaeda and secessionists in the south.
The Chinese oil men were kidnapped after a tribesman from the Laqmoosh clan was killed by police in a nearby checkpoint, reports said.
The governor of the region Ali Hassan al-Ahmadi said the kidnappers should give themselves up.
"They should release unconditionally their hostages and surrender," the defence ministry's information website said.