(CNN) -- The Movement for the Emancipation of the Niger Delta has claimed responsibility for an attack on an oil rig and the kidnapping of seven people.
MEND is one of several militant groups in the oil-rich Niger Delta that has been battling the government for years over the distribution of the country's oil wealth. CNN could not independently confirm the group's claim.
MEND said in a statement that its fighters carried out the attack on the Transocean rig off the southern coast of Nigeria at Okoro field.
"All the abducted expatriates are well and in our safe custody," the statement said. "All the fighters who participated in this attack have safely returned to base. Within the next 24 hours, we will release the names of all expatriates currently being held in our different camps."
London, England-based Afren PLC said two Indonesians, two Americans, two French and one Canadian were kidnapped. The Indonesians were not included in an earlier tally released by the company.
Two crew members are in stable condition at a shore-based clinic, where they were airlifted after being wounded.
State Department spokesman P.J. Crowley said the United States is in contact with Nigerian officials.
"We [are], of course, concerned about their safety and hope for their immediate release," he said. "We are working with Nigerian authorities ... to pursue their prompt release, and there's an investigation already under way."
Drilling operations in the Okoro field have been temporarily suspended, both companies said.
In a news release, Afren said a "security breach" occurred soon after its High Island VII jackup rig arrived but before drilling had commenced.
"The situation has not been resolved. Afren is doing everything it can to resolve the situation as quickly as possible, and it is working with relevant agencies in the Nigerian government," Afren spokesman James Henderson told CNN.
Transocean operates six offshore rigs in Nigeria -- two deepwater rigs, two ultra-deepwater rigs and two shallow-water rigs, including the one involved in the incident, said Guy Cantwell, a spokesman for the Houston, Texas-based company.
The company has "taken additional safety precautions" on its other assets in the west African nation, Cantwell said.
Nigerian officials would not comment about the incident.
CNN's John Dear and Vivian Kuo contributed to this report.