WARRI, (Nigeria): Four Chinese workers abducted from a quarry site in southern Nigeria have been freed, police said Sunday.

Unknown gunmen seized the Chinese nationals in Akpabuyo in Cross Rivers state last month and killed their police guard.

"We were able to secure the release of the Chinese nationals in Akpabuyo at about 1700 hours on Saturday," state police spokeswoman Irene Ugbo told AFP.

Ugbo could not say if ransom was paid. "They are currently receiving treatment at an undisclosed hospital and no arrest has been made yet," she said.

"I am not also aware of any payment of ransom for their release," she added.

Chinese firms are working in Nigeria on multi-billion dollar infrastructure projects that include railways, airports and roads. Their workers have been repeatedly targeted by kidnap gangs. Kidnapping for ransom is common in Nigeria, especially in the oil-rich south. The victims are usually released after a ransom is paid although police rarely confirm if money changed hands.