Authorities in Nigeria's central Niger state are searching for gunmen who attacked a mine this week and abducted several people, including four Chinese citizens. Nigerian media report the attack Wednesday killed an unknown number of workers. It's the second time this year that Chinese workers have been abducted in the state, as insecurity spreads in Nigeria.

Niger state Police Commissioner Monday Bala Kuryas said reinforcements have been sent to the Shiroro local government area, where the Ajata Aboki mining site is located.

Armed men attacked the site on Wednesday, opening fire on operators and killing an unspecified number while kidnapping workers, including four Chinese citizens.

Kuryas said the mining site is far into the bush and that some security operatives, including the military, police and local vigilantes, immediately responded to a distress call from the site and ran after the attackers.

He said four police officers were killed but did not disclose how many military personnel and vigilantes were affected. He also said security officials killed some of the armed men.

'For now, we're on their trail. Some of them were neutralized,' Kuryas told VOA. 'We're still on it trying to find out the exact number, that's the update for now, we're still investigating, the military I cannot speak for them.'

China's embassy in Nigeria has not made an official statement on the incident and was unavailable for comment Friday.

State Governor Abubakar Sani-Bello called the attack disturbing, and urged security officials not to relent in efforts to restore peace in the state.

Abuja-based security expert Patrick Agbambu said, given the record of attacks in the state, authorities should have been more vigilant.

'Knowing that Niger state has been a flashpoint of such attacks in recent times, I expected more security to be in place,' he said. 'Foreign nationals are considered to be more lucrative persons to be kidnapped foe ransom and for attention, it's going to become rampant and just maybe, Nigeria will become unsafe for business.'

Nigeria is seeing a wave of attacks in several regions and analysts say foreign workers and nationals are often targets of criminal gangs seeking to squeeze huge payments from employers.

In January, three Chinese nationals, working on a hydro-electric power project in Shiroro, were abducted.

Beijing has been warning nationals working in Nigeria to be vigilant in areas prone to attacks. In May, Chinese officials and representatives of local Chinese companies in Nigeria held talks on security matters.