Berekat Simon, a spokesman for the Ethiopian prime minister, said the attack was "a massacre. It is a terrorist act, ordered by a terrorist alliance that includes ONLF".
Simon was referring to the Ogaden National Liberation Front, a separatist group that has previously campaigned for independence for Somalis living in Ethiopia's Ogaden region.
In April last year the group warned international oil firms that exploring for oil and natural gas in the region "for the benefit of the Ethiopian regime or any foreign firm will not be tolerated".
The Xinhua news agency said the gunmen briefly took control of the working field after exchanging fire with more than 100 Ethiopian soldiers guarding the area.
A Chinese embassy spokeswoman confirmed the attack.
"I can confirm a number of people were killed, she said, asking not to be named.
"They are Chinese and Ethiopian. Some Chinese people have also been kidnapped. I cannot confirm numbers at the moment."
Chinese investment in African oil resources has increased dramatically in recent years in order to fuel its growing economy.