He said this after a meeting with Ukraine President Petro Poroshenko in Kyiv on Thursday, an Ukrinform correspondent reports.
"On the defensive lethal weapons, we are actively reviewing it," Mattis said.
He added that upon his return to the United States, he would inform the leadership of his country about the situation in Ukraine and put forwards his recommendations.
"I will go back now having seen the current situation and be able to inform the Secretary of State and the President in very specific terms what I recommend for the direction ahead," he said.
The Pentagon chief also said that Washington had recently decided to supply Ukraine with more than $175 million worth of equipment that will strengthen Ukraine's defense capability.