By Betsy Klein, CNN

(CNN) — US Secretary of State Rex Tillerson issued a harsh condemnation of Russia following a nerve agent attack on a former Russian spy in the United Kingdom last week, going further than the White House’s response stopping short of pinning blame for the attack on Moscow.

In a Monday morning call with United Kingdom Foreign Secretary Boris Johnson, Tillerson discussed the attempted murder of former Russian double agent Sergei Skripal and his daughter Yulia in the English city of Salisbury, according to a State Department statement.

“We have full confidence in the UK’s investigation and its assessment that Russia was likely responsible for the nerve agent attack that took place in Salisbury last week,” Tillerson said in the statement.

He continued, “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. From Ukraine to Syria — and now the UK — Russia continues to be an irresponsible force of instability in the world, acting with open disregard for the sovereignty of other states and the life of their citizens.”

The State Department’s position on the attack appears be much stronger than the White House’s response.

White House press secretary Sarah Sanders called the attack “reckless, indiscriminate and irresponsible,” but stopped short of blaming Russia.

“We’ve been monitoring the incident closely, take it very seriously,” Sanders said at Monday’s news briefing. “The use of a highly lethal nerve agent against UK citizens on UK soil is an outrage. The attack was reckless, indiscriminate and irresponsible. We offer the fullest condemnation, and we extend our sympathy to the victims and their families, and our support to the UK government. We stand by our closest ally and the special relationship that we have.”

Pressed further on whether Russia was behind the act, Sanders said, “Right now, we are standing with our UK ally.”

Earlier in the day, British Prime Minister Theresa May said Russia was “highly likely” responsible for the attack.

Speaking to reporters while traveling in Africa, Tillerson said the attack “clearly came from Russia” and would “certainly trigger a response.” Though it is still unclear to the State Department whether the Russian government was aware of the attack, the substance used in the attack was distinctive.

“This is a substance that is known to us and does not exist widely. It is only in the hands of a very, very limited number of parties. And I don’t want to say anything further than that,” Tillerson said.

Tillerson called for “appropriately serious consequences” for “both those who committed the crime and those who ordered it.”

The White House did not immediately respond to CNN’s request for comment on whether it accepts the State Department’s determination.

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