LONDON — British Prime Minister Theresa May says the Russian ex-spy poisoned in England was exposed to a military-grade nerve agent of type produced by Russia.

May told lawmakers during an address in parliament on Monday it was “highly likely’ Russia was responsible for poisoning Sergei Skripal, the former Russian military intelligence officer who was convicted of spying for Britain.

May says Russia’s ambassador to the U.K. has been summoned to explain how a Russian nerve agent turned up in Salisbury, the English city where Skripal and his adult daughter were sickened.

The British prime minister says if Moscow is proven to be behind the poisoning, her government will consider it an “unlawful use of force” by Russia.

Russian President Vladimir Putin says Britain should figure out what happened to Skripal before blaming the poisoning on Russia.

Asked by a British reporter in southern Russia if Russia was behind the poisoning, Putin said in comments carried by Russian news wires on Monday: “You first get to the bottom of things over there, and after that we can discuss it.”

Skripal is a former Russian military intelligence officer who was convicted of spying in Britain and released from prison as part of a spy swap.

He and his daughter remain in critical condition after the March 4 nerve agent attack in England. Authorities haven’t said what nerve agent was used.