Kenya’s national police force says 20 “militants” had been killed and eight officers injured in a suspected Al-Shabaab attack near the country’s border with Somalia.

A special unit of police officers came under “heavy” fire in an ambush on Wednesday while patrolling in Mandera, a county in north-eastern Kenya that shares a 700-kilometre frontier with Somalia.

Consequently, Nairobi has announced it will delay planned reopening its border with Somalia following a sudden increase in terrorist attacks on Kenyan territory by the Somali insurgents. Kenya’s Cabinet Secretary for Internal Security Kithure Kindiki announced on Thursday that Nairobi would put on hold planned free flow of goods, service and people across the border until border security was fully addressed and Al Shabaab pushed back.

The Wednesday gunfight “left 20 militants fatally injured” and eight officers injured, the national police service said on Thursday in a statement on its official Twitter account.

“Police also recovered assorted weapons from the scene of crime,” it added, alongside photographs of a heavy machine gun and rocket launchers.

On June 13, eight Kenyan police officers were killed in Garissa, an eastern county along the border with Somalia, when their vehicle struck an improvised explosive device. On June 24, five civilians had their throats cut in an attack in Lamu, another county on the border. Some were beheaded.

Kenya has suffered retaliatory attacks by Al-Shabaab since sending troops over the border into Somalia in 2011 to crush the jihadists who have been fighting to overthrow the foreign-backed government in Mogadishu since 2007.

Kenya remains a major contributor to an African Union force in Somalia trying to curb Al-Shabaab’s capacity to wage deadly attacks. In 2015, 148 people were massacred at Garissa University, and two years earlier 67 people were killed when militants stormed the Westgate mall in Nairobi.

• A Tell / AP report