MAIDUGURI, Nigeria (Reuters) - Two Chinese construction workers have been shot dead by unknown gunmen in Nigeria's northeast Borno state, the police said on Thursday, in a region plagued by an Islamist insurgency.

The killings bring to four the number of Chinese workers killed in the last month in Borno, after gunmen suspected of being robbers killed two construction workers in October.

Radical Islamist sect Boko Haram has killed hundreds in bomb and gun attacks in Borno state since it launched an uprising there in 2009, mostly targeting figures of authority.

"At about 8 a.m. (Wednesday) some gunmen shot and killed two Chinese construction workers at Benisheik in Kaga local government of Borno state," Police Commissioner Abdullahi Yuguda told Reuters.

He said a policeman was killed and several officers injured in Banki, another area in Borno, on Wednesday when they were attacked by gunmen.

Boko Haram's insurgency has contributed to a breakdown in order across the north of Africa's most populous nation and security sources believe criminal gangs carry out deadly armed robberies knowing Boko Haram is likely to be blamed.

Styled on the Afghan Taliban, the sect's purported leader Abubakar Shekau has said he wants to impose sharia, Islamic law, on the country of 160 million people, around half of whom are Christians and the other half Muslim. His movement has become the number one security threat to Africa's top energy producer.

Western governments are increasingly concerned about the links Boko Haram has with outside groups, including al Qaeda's north African arm.

(Reporting by Ibrahim Mshelizza; Writing by Joe Brock; Editing by Michael Roddy)