A suicide car bomb struck near a military base in Afghanistan's Khost province on Monday, Afghan news agency Khaama Press reported.

The Afghan interior ministry confirmed that a car bomb went off near a military base at around 1pm local time.

The attackers had detonated a car bomb at an entrance to Camp Chapman, a secretive facility manned by US forces and private military contractors, said Mubarez Mohammad Zadran, a spokesman for the provincial governor.

“I am aware of a car bomb attack at one of the gates in the US base, but we are not allowed there to get more details,” the spokesman said.

A spokesman for the US military in Afghanistan, Capt. William Salvin, confirmed the car bomb attack.

He said there appeared to be a number of Afghan casualties but none among U.S. or coalition personnel at the base.

The incident came as US Defence Secretary James Mattis arrived in Afghanistan on an unannounced visit Monday, an American defence official confirmed, hours after his Afghan counterpart resigned over a deadly Taliban attack.

Mattis, making his first visit to Afghanistan as Pentagon chief, was due to meet top officials including President Ashraf Ghani less than two weeks after the US dropped its largest non-nuclear bomb on militant Islamic State (IS) hideouts in the country's east.

He arrived as embattled Afghan security forces faced chaos with the resignations of defence minister Abdullah Habibi and army chief Qadam Shah Shaheem.

The resignations, announced in a terse one-line statement from the presidential palace, came amid fury over the Taliban assault on an army base outside the northern city of Mazar-i-Sharif on Friday.

Ten gunmen dressed in soldiers' uniforms and armed with suicide vests entered the base in army trucks and opened fire at unarmed troops at close range in the mosque and dining hall.

It is believed to be the deadliest-ever Taliban attack on an Afghan military target, though the exact toll from the assault remains unclear.

Afghan officials have so far ignored calls to break down the toll it has given of more than 100 soldiers killed or wounded, but have been known to minimise casualties in such attacks in the past.

The US has said that at least 50 soldiers were killed, and some local officials have put the number of dead alone as high as 130.

The raid underscores the Taliban's growing strength more than 15 years since they were ousted from power, and as they gear up ahead of the spring fighting season.