Taliban insurgents attacked a U.S.-operated base in Afghanistan's eastern province of Khost on Monday officials said cited by Reuters, giving few immediate details of an assault that coincided with a surprise visit to Kabul by U.S. Secretary of Defense James Mattis. The attackers detonated a car bomb at an entrance to Camp Chapman, a secretive facility manned by U.S. 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 U.S. base, but we are not allowed there to get more details," Zadran said. A spokesman for the U.S. 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 attack came just three days after more than 170 Afghan soldiers were killed in an attack on their base by Taliban fighters disguised in military uniforms last Friday. On Monday, Afghanistan's defense minister and army chief staff resigned following the attack, Reuters reported.

Photos on social media showed a plume of smoke rising from the scene of the attack in the eastern province of Khost.

#BREAKING - explosion reported in Khost province, close to Khost airport close to U.S Forces base #Afghanistan pic.twitter.com/487W08xWHm — TOLOnews (@TOLOnews) April 24, 2017



General John Nicholson, the top American commander in Kabul, recently told Congress that he needs a few thousand more soldiers deployed to assist Afghan security forces, so they can eventually tackle the Taliban insurgency on their own.

Meanwhile, Defense Secretary James Mattis arrived in Afghanistan on Monday on a surprise visit the WSJ reported, and will meet with Afghan officials. In Kabul, Mattis will discuss war needs with both with Afghan officials and the top commander of the U.S.-led coalition, according to USA Today.

Mattis’s visit comes amid turmoil in the Afghan armed forces, and takes place just hours after the resignations of his Afghan counterpart and the army chief of staff following the deadliest attack by insurgents on government forces since the war began in 2001. Friday’s Taliban attack on a government army base in Balkh province left about 170 people dead, Afghan officials said, after six Taliban fighters infiltrated the heavily guarded base aboard military vehicles and opened fire in what became a five-hour battle. Five of the militants were killed and a sixth was captured alive by Afghan commandos who had been rushed to join the battle, according to Afghan military officials.

“There is a mosque and a dining facility on the base that seem to, at this point from our reports, have been the subject of significant attack from enemy forces,” U.S. Central Command spokesman Col. John Thomas told reporters at the Pentagon.

Last week, a report surfaced that Mattis had privately cast doubt on President Trump's proposed Pentagon spending levels, telling members of Congress the $603 billion request is not enough to fulfill Trump's vow to rebuild the military. Mattis has not spoken out publicly against the spending plan, but he told lawmakers privately that even with the proposed funding increase, the Defense Department would not receive enough to do everything Trump has proposed.

Currently, there are about 8,500 U.S. forces and some 6,000 soldiers from other members of the international coalition in Afghanistan in support of the central government in Kabul, which is fighting both the Taliban, the largest insurgency, and the local affiliate of the extremist group Islamic State.