WASHINGTON, March 12 (Xinhua) -- The U.S. government on Monday warned of a possible "response" to the poisoning of a former Russian spy and his daughter in Britain, as the White House and the State Department lashed out at Russia amid hostility over the issue.

White House Spokesperson Sarah Huckabee Sanders said at a news briefing that the attack was "reckless, indiscriminate, and irresponsible."

Noting that the United States has been monitoring the incident closely and taking it "very seriously," Sanders said the use of a highly lethal chemical nerve agent against British citizens on British soil is "an outrage."

"We stand by our closest ally and the special relationship that we have," she referred to the British government, adding that the United States is ready "if we can be of any assistance to them."

Also on Monday, U.S. Secretary of State Rex Tillerson said the poisoning "came from Russia."

Tillerson told the media on his way back from Nigeria to Washington that although he did not know whether the Russian government knew the poisoning, the tragedy could not have originated anywhere else, as the substance was "only in the hands of a very, very limited number of parties."

Noting that the poisoning was "a really egregious act" that was "almost beyond comprehension," Tillerson said that after a year of trying, "we didn't get very far" in the efforts to narrow down differences with Russia.

The U.S. top diplomat added "it certainly will trigger a response" if the Russian government is proved behind the incident.

Later on the day, Tillerson issued a statement saying that he spoke with his British counterpart Boris Johnson in the morning.

"We have full confidence in the UK's investigation and its assessment that Russia was likely responsible for the nerve agent attack," he said. "There is never a justification for this type of attack ... and we are outraged that Russia appears to have again engaged in such behavior."

He also accused Moscow of being "an irresponsible force of instability" in issues of Ukraine, Syria and now Britain, warning that those who are responsible "must face appropriately serious consequences."

The statements of the White House and the State Department came on the heels of an announcement made by British Prime Minister Theresa May, who on Monday also pointed her finger at Russia over the developing "poisoning attack" row.

Claiming that Moscow was "highly likely" responsible for the attack, May told the parliament that Russia's ambassador in London had been summoned to explain whether it was "a direct action by the Russian state" or the result of it "losing control" of its stock of nerve agents.

May said the attack, targeting Sergei Skripal and his daughter Yulia in London on March 4, involved a military-grade nerve agent of a type developed by Russia.

Her comments came after Skripal and his daughter, who remain in a critical but stable condition, had been found slumped on a bench in London.

On Monday, the Kremlin's spokesman Dmitry Peskov denied Russian involvement in the incident.

Russian Foreign Ministry Spokeswoman Maria Zakharova also said May's statement about the alleged Russian trace in the attack was a "circus."

"The conclusion is obvious: it is another informational and political campaign based on a provocation," Zakharova was quoted by Russian media outlet Sputnik as saying.