BEIJING, February 4 -- Nine Chinese oil workers who were kidnapped last week by an armed group in Nigeria, were released safely on Sunday morning, according to the Chinese Foreign Ministry.

The nine were set free at 6:35 a.m. (Beijing time) following 11 days of tough diplomatic efforts, Foreign Ministry spokeswoman Jiang Yu said in a statement.

The released workers will return to China soon, she added.

The nine are in stable physical condition and currently resting at their base in Port Harcourt, the capital of Nigeria's southern Rivers State, said Zhao Enhui, the managing director of the Chinese company.

A Chinese oil company compound was attacked by an unidentified armed group in southern Nigeria on January 25. Nine workers were kidnapped and the others moved to a safe place.

The Chinese government showed deep concern about the workers and demanded relevant departments investigate the case and go all out to rescue the kidnapped workers, said Jiang.

The Chinese Foreign Ministry, together with the Ministry of Commerce and Chinese diplomatic missions in Nigeria and elsewhere, had worked with the Chinese company to carry out the rescue work, according to Jiang.

The Chinese government also asked for evaluation of security conditions and precaution measures to ensure the safety of Chinese nationals, she said.

"We extend our sincere greetings to the nine workers, and express our appreciation of the assistance from the Nigerian government and other sides for the rescue work," she said.

The attack happened four days after five Chinese telecom workers who had been kidnapped in Nigeria returned to China. They were captured by unidentified armed men on January 5 in the southern Nigerian state of Rivers and released 13 days later on January 17.

A spate of attacks and kidnappings on foreign companies and workers have occurred in Nigeria, the largest oil producer in Africa and the sixth largest oil exporter in the world with a daily output of 2.5 million barrels.

"We condemn the recent attacks and kidnappings and demand an end of such actions," Jiang said.

Meanwhile, the spokesperson also called for the overseas Chinese companies and nationals to further enhance their awareness of safety and strengthen security measures to ensure their safety.

Also on Sunday, an armed group of Nigeria which had kidnapped 24 Filipino seamen, called "the Movement for the Emancipation of Niger Delta", warned that they would continue to hold the hostages until all their demands are met by the Nigerian government.