KABUL - Mattis is the first member of US President Donald Trump’s cabinet to visit the country since Trump pledged to stay the course in America’s longest war.
In a sign of the nation's continuing insecurity a volley of rockets landed on a house near Kabul international airport hours after Mattis flew in, the interior ministry said.
One person died and four others were wounded -- all from the same family -- in the assault claimed by the Taliban, which said on social media the missiles had been aimed at Mattis's plane.
The Islamic State's local Khorasan province affiliate also claimed responsibility, as security forces were locked in a stand-off with the attackers.
The unannounced high-level visit came as Afghanistan's beleaguered security forces struggle to beat back the Taliban, which has been on the offensive since the withdrawal of US-led NATO combat troops at the end of 2014.
Mattis, along with Stoltenberg, was to hold talks with President Ashraf Ghani and other top officials to discuss the US-led NATO "train and assist" mission -- designed to strengthen Afghanistan's military so it can defend the country on its own.
At a joint news conference with Ghani at the presidential palace Mattis and Stoltenberg pledged the support of US and NATO allies to the Afghan conflict, and expressed determination to stop the country becoming a safe haven for terrorists.
The foreign assistance would give Afghan forces a "compelling battlefield advantage over anything the Taliban stands to mass against" it, Mattis told reporters.
"We will not abandon Afghanistan to a merciless enemy trying to kill its way to power."
Stoltenberg said: "The more stable Afghanistan is the more safe we will be," adding that more than 15 NATO members had agreed to send additional troops.
US generals have for months been describing the situation in Afghanistan as a stalemate, despite years of support for Afghan partners, continued help from a NATO coalition and an overall cost in fighting and reconstruction to the United States of more than $1 trillion.
October marks the 16th anniversary of the start of the war. America is pressing NATO partners to increase their own troop levels in the country to help Afghan forces get the upper hand in the grinding battle against the Taliban and Islamic State.
The resurgent Taliban have promised to turn Afghanistan into a "graveyard" for foreign forces and have been mounting deadly attacks as they maintain their grip on large swathes of the country.
On allegations that Iran and Russia are actively propping up the Taliban, Mattis said it would be "extremely unwise if they think they can somehow support terrorism in another country and not have it come back to haunt them".
- Slapdash training -
Under Trump's plan, the US is sending more than 3,000 additional troops to Afghanistan, on top of the 11,000 already on the ground, to train and advise the country's security forces.
NATO allies have around 5,000 troops deployed around the country.
Critics have questioned what the extra US soldiers can accomplish that previous forces -- who numbered some 100,000 at the height of the fighting -- were unable to do.
Earlier this year Ghani ordered a near-doubling of the country's Special Operations Command -– the elite fighting force spearheading Afghanistan's war against insurgents -- from 17,000 as part of a four-year plan that also aims to strengthen Afghanistan's air force.
While Afghan authorities have welcomed Trump's open-ended commitment to increase US troop numbers, they know it will take time to improve the fighting abilities of their own forces.
Afghanistan's soldiers have been severely demoralised and weakened by huge casualties, desertions and corruption.
Earlier this week US watchdog agency SIGAR criticised US efforts to train Afghanistan's security forces, saying they were being hampered by slapdash instruction, shoddy oversight and failures in governance.
Mattis flew to Kabul from New Delhi where he met his Indian counterpart Nirmala Sitharaman, who made it clear that India would not deploy troops to Afghanistan as part of Trump's strategy. (AFP)