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.”
The Trump administration’s relationship with President Vladimir V. Putin has been contradictory, with President Trump often complimenting the Russian leader while Mr. Tillerson has become increasingly critical.
On Monday night, the White House directed reporters to a formal statement from the State Department, which said: “There is never a justification for this type of attack — the attempted murder of a private citizen on the soil of a sovereign nation — and we are outraged that Russia appears to have again engaged in such behavior.”
Mr. Tillerson spoke to reporters while on a flight from Nigeria to Cape Verde. Hours earlier, Britain’s prime minister, Theresa May, said it was “highly likely” that Russia was to blame, and she demanded answers from the Kremlin.
Mr. Tillerson said that he had just spoken with his British counterpart, Foreign Secretary Boris Johnson, and that the State Department would issue a statement affirming the British findings.
“This is a really egregious act,” Mr. Tillerson said. “It appears that it clearly came from Russia. Whether it came from Russia with the Russian government’s knowledge is not known to me at this point.”
He added that he would be stunned if a government was behind the use of a deadly nerve agent. The former spy, Sergei V. Skripal, once an informant for Britain’s foreign intelligence service, and his daughter, Yulia, were found unconscious on a park bench in of Salisbury, England, and a police officer who helped them has also been hospitalized, in serious condition.
“It’s almost beyond comprehension that a state, an organized state would do something like that,” Mr. Tillerson said in the interview. “A nonstate actor, I could understand. A state actor — I cannot understand why anyone would take such an action.”
He noted that the nerve agent used in the attack “is only in the hands of a very, very limited number of parties.” The Organization for the Prohibition of Chemical Weapons, a watchdog agency, is also looking into the attack.
Asked whether the attack would prompt a response in defense of Britain, a NATO ally that the United States is legally obligated to defend if it came under attack, Mr. Tillerson said: “It certainly will trigger a response. I’ll leave it that.”
Mr. Tillerson cut short his Africa trip a day earlier than expected to start what he described as the intense homework for President Trump’s meeting with the North Korean leader, Kim Jong-un.
In a wide-ranging interview aboard his plane, Mr. Tillerson said he left behind a stronger relationship with President Idriss Déby of Chad, who was angered when Chad was placed on a list of countries whose citizens are virtually barred from entering the United States.
Mr. Tillerson appeared exhausted on the flight home Monday. He had left the United States on the evening of March 6, but after just a day of meetings in Addis Ababa, Ethiopia, he woke at 2:30 a.m. on Thursday with the news that President Trump had decided to accept Mr. Kim’s invitation for talks.
He had another urgent phone call Friday night — he would not say about what — so that by Saturday he had managed to get just four hours of sleep over three days, he said. And then he got food poisoning.
“So the combination of that, I just said, ‘This is inhumane,’” he said with a chuckle.
Mr. Tillerson canceled his meetings set for Saturday in Nairobi, including a visit to an H.I.V./AIDS clinic, and he decided to fly home Monday night after quick visits to Chad and Nigeria.
The potential North Korean summit meeting dominated much of Mr. Tillerson’s week even though he was halfway around the world from Washington. In a news conference Monday, Mr. Tillerson said that planning for the event was in “very early stages.”
But he said in his later interview on his plane that, in his own experience as the chief executive of oil giant Exxon Mobil, intense preparation — what he referred to as “homework” — was needed to make such a meeting successful.
Without good preparation, he said, “it’s very difficult to map a way forward.”
Early on Monday, Mr. Tillerson spent several hours in Ndjamena, Chad, where he insisted that the people of Chad were “welcome in the United States.”
He said that Chad’s efforts to strengthen its passport controls and increase information sharing may result in its removal from the travel ban.
Chad’s foreign minister, Mahamat Zene Cherif, called the inclusion of Chad on the travel ban an “injustice,” and said that widely reported disparaging remarks attributed to Mr. Trump about Africa “shocked almost all Africans.”
State Department and Pentagon officials wanted to keep Chad — a partner in the fight against terrorism in the Sahel — off the travel ban list when it was formulated but were overruled by the White House. Mr. Tillerson said he hoped the country’s inclusion would soon be corrected.
At his last stop on the continent, in Abuja, Nigeria, Mr. Tillerson met with President Muhammadu Buhari of Nigeria and then held a news conference with Foreign Minister Geoffrey Onyeama. They discussed cooperation on trade and in the fight against Boko Haram and an offshoot of the Islamic State.