Tillerson made the assertion at a press conference held in Kuwait, where he is attending a high-level meeting between members of a US -led coalition against the Daesh terrorist group.
Earlier in the day, Kuwait's ruling emir said his oil-rich nation will give $1 billion in loans and $1 billion in direct investments to help rebuild Iraq.
Contributions came in two forms, loans, where countries are giving to Iraq reconstruction projects, and investments from both the private and government sector.
"This large assembly of worldwide communities that are here today is reflective of the large loss that Iraq withstood in facing terrorism", the Kuwaiti emir said.
"Iraq can not commence the mission of rebuilding itself without support, which is why we are all here today from all around the world, to stand by Iraq's side", Sheikh Sabah said.
Luaibi said the downstream investment would lift refining capacity to 1.5 million bpd by 2021, with 500,000 bpd of that earmarked for export.
Saudi Arabia and Qatar also said they would allocate $1.5 billion and $1 billion, respectively.
The Islamic Development Bank also pledged $500 million toward reconstruction in Iraq.
"The whole world owes you a debt for your struggle against the deadly global threat posed by Daesh (ISIL)", Guterres said, in comments directed at the Iraqi delegation, as quoted by AFP.
"We will also provide project support of 50 million dollars to Iraq". The New York Times has suggested failure to obtain the vast sums needed for reconstruction represents "a humiliating blow for the Iraqi government" and could threaten the future of Iraq's Prime Minister, Haider Al-Abadi.
"There are more than five million internally displaced persons in Iraq".
Overall, Iraq is seeking $US88.2 billion in aid from donors.
It took over three years for Iraqi armed forces to fully liberate the country's territory from the clutches of Daesh terrorists.