Argentina's President Mauricio Macri (R) and President Barack Obama leave after posing at the Casa Rosada government house in Buenos Aires, March 23, 2016.

The world must unite to fight terrorism and the United States can and will defeat Islamic State, U.S. President Barack Obama said on Wednesday.

Obama, speaking at a news conference with Argentina's president, Mauricio Macri, referred to Tuesday's attacks in Brussels and said: "The United States will continue to offer any assistance that we can to help investigate these attacks and bring the attackers to justice."

"We will also continue to go after ISIL aggressively until it is removed from Syria and removed from Iraq and is finally destroyed," he added, referring to Islamic State by an acronym.