Three Chinese warships are visiting Nigeria as part of growing naval cooperation between the two countries, officials said on Monday.

The visit comes after US defence officials last year expressed concern over Chinese plans for a military base on West Africa’s Atlantic coast.

China is a major investor in infrastructure projects in Nigeria, including in the recently opened Lekki deep sea port near the economic capital Lagos.

The Gulf of Guinea – a major shipping route stretching 5,700 kilometres (over 3,500 miles) from Senegal to Angola – has long been at risk from piracy with Nigerian gangs carrying out most attacks on foreign shipping vessels.

China’s Escort Task Group 162 compromising three ships arrived in Nigeria after escorting a Chinese merchant vessel from the Gulf of Aden, according to Nigeria’s navy.

“They are on port call on completion of task before heading home. The visit is to improve diplomacy,” Nigeria’s navy spokesman Commodore Ayo Vaughan said.

“They hope the visit will strengthen ties between the two navies.”

One of the three ships, the destroyer Nanning, berthed in Lagos and the three will leave on Wednesday, according to the Chinese embassy in Nigeria.

Last year, General Stephen J. Townsend, commander of the US Africa Command, told Congress the US military was concerned about China opening a naval base on Africa’s coast.

“As a first priority, we need to prevent or deter a Chinese space on the Atlantic coast of Africa,” he said, according to a US Department of Defense statement.

The United States is one of the major arms suppliers for Nigeria, including recently for aircraft it uses in its long conflict with jihadists in the country’s North-East.