The US is speaking with authorities in Iraq and the regional Iraqi Kurdistan government to help improve their missile defence capabilities after an attack by Iran.
On Sunday, Iran launched up to 12 ballistic missiles aimed at a US consulate complex under construction in Erbil, in an attack described by Iraq's president as an act of “terror".
Mr Sullivan said no US citizens were harmed in the attack and no American installations were hit. He told TV network CBS the US would do whatever it takes to defend its people, interests and allies.
“We are in consultation with the Iraqi government and the government in Iraqi Kurdistan, in part to help them get the missile defence capabilities to be able to defend themselves in their cities,” Mr Sullivan said.
Iran's Revolutionary Guards claimed responsibility for the attack in the early hours of Sunday, Iranian state media reported.
The missiles were aimed at the US consulate among other sites, said the Kurdish regional government.
Mr Sullivan, in a subsequent statement released by the White House, said the US backs Baghdad and governments across the region in the face of threats from Tehran.
“We will support the government of Iraq in holding Iran accountable, and we will support our partners throughout the Middle East in confronting similar threats from Iran,” he said.
Asked about the attack's impact on negotiations aimed at signing a nuclear deal with Iran, which are already at an impasse, Mr Sullivan said: “The various negotiators are back home in their capitals and we will have to see what happens in the days ahead with respect to the diplomacy around the nuclear deal.”
He said President Joe Biden remained strongly committed to stopping Iran from acquiring nuclear weapons.
“One thing I will say is that the only thing more dangerous than Iran armed with ballistic missiles and advanced military capabilities is an Iran that has all of those things and a nuclear weapon,” he said.
US forces stationed at the Erbil International Airport complex have previously come under fire from rocket and drone attacks that Washington blamed on Iran-aligned militia groups, but no such attacks had occurred for several months.
The US Department of Defence referred all queries about Mr Sullivan's comments on missile defence capabilities to the State Department, which oversees security assistance agreements with other countries.