Russia will only co-operate 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 March 4 attack in the English city of Salisbury that 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.

"We have already made a statement to say this is nonsense," he said. "We have nothing to do with this."

Neil Basu, Scotland Yard's national co-ordinator with the Metropolitican Police, said investigators would like to speak to anyone who saw Sergei Skripal's red BMW in the Salisbury area between 1 p.m. and 4 p.m. on Sunday, March 4. (Toby Melville/Reuters )

He said Moscow is willing to co-operate 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.

More on pair's whereabouts

Neil Basu, Scotland Yard's national co-ordinator with the Metropolitan Police force, said where and how the poison was administered remain the investigation's key focus and he asked for the public's help.

Police cordoned off the upper level of a Sainsbury's supermarket parking lot, opposite the park bench where Skripal and his daughter were found unconscious on March 4. The two remain critically ill after being exposed to a nerve agent believed to be from the so-called Novichok group. (Chris J Ratcliffe/Getty Images)

He told reporters Skripal's daughter arrived at London's Heathrow Airport at 2:40 p.m. from a flight from Russia on March 3, the day before she and her father became ill.

The next afternoon, the two went to the upper level of the parking lot of Sainsbury's supermarket in Salisbury. Next, they stopped at The Hill pub, before heading to Zizzi, a nearby Italian restaurant, at approximately 2:20 p.m. They left Zizzi at 3:35 p.m.

Emergency services were called to the town centre at 4:15 p.m. and the pair were found "in an extremely serious condition on a park bench outside Zizzi restaurant."

'Highly likely' Russia involved

British Prime Minister Theresa May told the British House of Commons on Monday that Russia's involvement is "highly likely." Officials said Tuesday that May is reviewing a range of economic and diplomatic measures in retaliation for the poisoning.

May said British officials had identified the substance as being part of the Novichok group of nerve agents, which were developed by the Soviet military during the 1970s and 1980s.

Theresa May told legislators that Sergei Skripal was exposed to a military-grade nerve agent. 0:28

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 how the nerve agent came to be used.

Russian officials and media have made a variety of accusations against Britain in recent days, including seeking to influence Sunday's election, in which President Vladimir Putin is expected to win re-election.

14 deaths linked to Russia?

Meanwhile the cases of other Russians who have died under mysterious circumstances are being raised. British Home Secretary Amber Rudd said police and the domestic security service will look into 14 deaths in Britain that might be linked to Russia.

"In the weeks to come, I will want to satisfy myself that the allegations are nothing more than that," Rudd said. "The police and MI5 agree and will assist in that endeavour."

British Prime Minister Theresa May gave Russia a deadline of midnight Tuesday night to explain how an ex-spy was poisoned with a Soviet-era nerve agent. (Reuters)

BuzzFeed News reported in 2017 that 14 deaths in Britain and the U.S. dating to 2006 may have been linked to Russia. Among them are prominent critics of Russian President Vladimir Putin, including oligarch Boris Berezovsky and whistleblower Alexander Perepilichny.

British Foreign Secretary Boris Johnson said Tuesday that Britain is talking to its international partners about the poisoning cases, which include a police officer who responded to the medical emergency in Salisbury and remains in serious but stable condition.

"I've been encouraged by the willingness of our friends to show support and solidarity," he said.

"I think in particular from President (Emmanuel) Macron of France, I talked to Sigmar Gabriel my German counterpart, and from Washington where Rex Tillerson last night made it absolutely clear that he sees this as part of a pattern of disruptive behaviour ... malign behaviour by Russia ... the support for the reckless use of chemical weapons which stretches from Syria now to the streets of Salisbury."

U.S. Secretary of State Rex Tillerson told reporters late Monday that Russia's actions would "certainly trigger a response." He said it was "almost beyond comprehension" that a government would use such a dangerous substance in a public place.

'Must be held accountable'

The chief of the world's chemical weapons watchdog also said that those responsible "must be held accountable."

In a speech Tuesday to the Executive Council of the Organization for the Prohibition of Chemical Weapons, Director-General Ahmet Uzumcu said Johnson called him Monday evening to inform him of the results of investigations.

"It is extremely worrying that chemical agents are still being used to harm people. Those found responsible for this use must be held accountable for their actions," he said.

Skripal, a former Russian military intelligence officer, was convicted of spying for Britain and then released in a spy swap. He had been living under his own name in the small city of Salisbury for eight years before the attack without attracting any public attention.

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