Military forces work on a van in Winterslow, England, March 12, 2018, as investigations continue into the nerve-agent poisoning of Russian ex-spy. (Photo: AP)

Russia will only cooperate with Britain on the investigation into last week’s poisoning of an ex-Russian spy and his daughter if it receives samples of the nerve agent that is believed to have been used, Russia’s foreign minister said Tuesday.

Sergey Lavrov spoke in response to the British government’s demand for an explanation of the use of a military-grade nerve agent produced in Russia in the attack in the English city of Salisbury, which left 66-year-old Sergei Skripal and his 33-year-old daughter Yulia in critical condition.

Lavrov told reporters on Tuesday Moscow’s requests to see samples of the nerve agent have been turned down, which he called a violation of the Chemical Weapons Convention, which outlaws the production of chemical weapons. He insisted that Russia is “not to blame” for the poisoning.

He said Moscow is willing to cooperate with the probe but suggested that London would be “better off” complying with its international obligations “before putting forward ultimatums.”

It was not immediately clear if Lavrov’s comments constituted Russia’s official response to Britain’s demand for an explanation of the poisoning by the midnight Tuesday deadline.

Russian news agencies reported that the Foreign Ministry on Tuesday summoned the British ambassador in Moscow over the poisoning.

British Prime Minister Theresa May has said Russia’s involvement is “highly likely.” Officials said Tuesday May is reviewing a range of economic and diplomatic measures in retaliation for the assault with a military-grade nerve agent.

May’s conclusion, based on assessment from the police and intelligence services, is leading to a major confrontation between Britain and Russia, which has taken an increasingly aggressive posture toward Europe in recent years.

The prime minister says Russia has until the end of Tuesday to explain its actions in the case.