Another nine people were wounded, including eight civilian contractors and one U.S. soldier, he said.
The latest attack came amid rising tensions in north Iraq, where neighbouring Turkey has been waging a fierce land and air assault against the Kurdistan Workers' Party (PKK) since June 2020.
It's the first time in 2 months that U.S. installations have been targeted in Iraq.
More than a dozen rockets were fired toward the Erbil International Airport in Iraq's semi-autonomous Kurdish region, with three directly hitting a military base housing foreign troops at the airport.
Marotto also said a USA military serviceman and eight civilian contractors were wounded in the assault.
It was the most deadly attack to hit US-led forces in nearly a year in Iraq, where tensions have escalated between U.S. forces, their Iraqi and Kurdish allies on one side and Iran-aligned militias on the other.
One US service member was also injured, he said.
Tension between the United States and Iran sent the Middle East to the edge of a full-scale confrontation in January 2020 after a U.S. drone strike killed Iranian military mastermind Qassem Soleimani in Baghdad.
Days after that strike, the Iranian government launched missile attacks against United States forces at the Ain al Assad air base in Iraq's Anbar province, wounding more than 100 troops.
Since Iraq declared victory over the Islamic State group in late 2017, the coalition presence has been reduced to fewer than 3,500 troops, 2,500 of them American.
Kurdish counter-terrorism forces said they had found the vehicle the rockets were launched from but did not say where it was discovered.
A little-known Shiite militant group calling itself Saraya Awliya al-Dam, Arabic for Guardians of Blood Brigade, claimed responsibility for the attack. No one immediately claimed responsibility.
Ashab Ahl al-Kahf has claimed previous rocket attacks on United States bases but is thought by Iraqi and Western officials to be a "smokescreen" for other groups.
Smoke rises over Irbil on Feb 15, 2021.
US Secretary of State Antony Blinken called Monday for an investigation into a rocket attack on an airbase in Iraq's Kurdistan region and promised to "hold accountable those responsible".
"This is a test of the new Biden administration to see what they can get away with", Knights said in a telephone interview.
Before leaving office in January, Trump had threatened that any further fatal attacks would prompt a mass bombing campaign, with Iraqi sources telling AFP that more than 100 sites would be targeted.
The rocket attack occurred Monday around 9:30 PM local time, and claimed the life of a civilian contractor and injured six others, according to Al-Arabiya.
Egypt condemned a deadly rocket attack that targeted the city of Erbil in Iraq, offering condolences to the government and families of victims a statement by the Ministry of Foreign Affairs said on Tuesday. The embassy in Baghdad continues to operate with the ambassador and a small number of key staff.