SANTA MARIA, Cape Verde — Secretary of State Rex W. Tillerson on Monday called the poisoning of a former Russian spy in Britain “an egregious act” and added, “It appears that it clearly came from Russia.”

The statement, made in an interview with reporters at the end of a five-nation tour of Africa, was the clearest statement yet from the Trump administration, after several days of equivocation in which American officials declined to explicitly blame Russia for the March 4 attack.

“I’ve become extremely concerned about Russia,” Mr. Tillerson said in the interview. “We spent most of last year investing a lot into attempts to work together, to solve problems, to address differences. And quite frankly, after a year, we didn’t get very far. Instead what we’ve seen is a pivot on their part to be more aggressive.”

He added: “And this is very, very concerning to me and others, that there seems to be a certain unleashing of activity that we don’t fully understand what the objective behind that is. And if in fact this attack in the U.K. is the work of the Russian government, this is a pretty serious action.”