A Kenyan court has ordered four British aviation enthusiasts to pay a fine or face a year in jail for trespassing and taking pictures without permission at Nairobi's Wilson Airport, one of their lawyers said.

The four plane spotters were detained on March 12 in Kenya, which has tightened security around official buildings, hotels and other sites after a spate of attacks by al Shabaab Islamists.

"The four British nationals were convicted on their own plea of guilty," lawyer Nicodemus Ouma told Reuters.

His firm was helping work out arrangements to pay the fines of 200,000 Kenyan shillings ($1,970) for each of them.

The men were convicted of trespassing in a restricted area and taking pictures without permission.

Wilson Airport, in the capital Nairobi, handles mostly domestic and chartered flights. Jomo Kenyatta International Airport is the capital's main airport and lies on the outskirts of the city.