Britain demanded answers from Russia Thursday after a couple was exposed to the same nerve agent used on a former Russian spy and his daughter in an attempted murder blamed on Moscow.
But Russia quickly hit back, denouncing Britain for playing "dirty political games" and demanding London apologise.
The British couple fell ill on Saturday in Amesbury, a small town near the southwestern English city of Salisbury where former double agent Sergei Skripal and his daughter Yulia collapsed on March 4.
That incident triggered a major diplomatic crisis with Russia after Britain and its allies accused Moscow of trying to kill them -- a charge strongly denied by the Kremlin.
Speaking to parliament on Thursday, Interior Minister Sajid Javid said a link between the cases was "clearly the main line of inquiry" and demanded Moscow explain itself.
"It is now time that the Russian state comes forward and explains exactly what has gone on," he said, noting the global focus on Russia as it hosts the football World Cup.
"It is completely unacceptable for our people to be either deliberate or accidental targets, or for our streets, our parks, our towns to be dumping grounds for poison."
But his remarks sparked a sharp response from Moscow which suggested that the British police steer clear of involvement in political intrigues.
"We urge British law enforcement not to get involved in dirty political games that certain powers in London have already begun and instead finally cooperate with Russian law enforcement in their investigations," foreign ministry spokeswoman Maria Zakharova told reporters.
And the British government owed Russia an apology, she said.
"This government and its representatives will have to apologise to Russia and the international community," she said, adding: "It will happen."
-EXPOSED TO NOVICHOK -
Police said tests on the couple, Dawn Sturgess, 44, and Charlie Rowley, 45, revealed they were exposed to Novichok, but it was not clear if it was the same batch used on the Skripals.
Novichok is a military-grade nerve agent developed by the Soviet Union during the Cold War.
Kremlin spokesman Dmitry Peskov described the case as "very worrying" but said Russia had no information "about what substances were used and how they were used".
"From the very beginning, the Russian side proposed conducting a joint investigation with the British side and this proposal remained without a response," he said.
Javid chaired an emergency cabinet meeting earlier Thursday, attended by the defence and foreign ministers among others.
Speaking in Berlin where she was holding talks with German Chancellor Angela Merkel, Prime Minister Theresa May described the incident as "deeply disturbing" and pledged the police "will be leaving no stone unturned in their investigation."
- SIX SITES SEALED OFF -
Around 100 counter-terror officers have joined the police investigation, and six sites in Salisbury and Amesbury that were visited by the couple before they collapsed have been cordoned off.
Officials insist there is no significant risk to the wider public, but urged anyone who had visited the affected sites wash their clothes and wipe down personal items.
Many questions remain over the source of the contamination -- and why tests were not conducted on the couple until Monday, two days after they became ill.
One theory being considered in government is that one of them picked up the container which stored the nerve agent used against the Skripals.
The Novichok was believed to have been smeared on the Russians' front door, and the container could have been discarded somewhere in Salisbury.
Police say there is no evidence the latest victims recently visited any of the sites linked to the Skripals, which have since been decontaminated.
But one of their friends, Sam Hobson, said they had visited Salisbury on Friday.
- FOAMING AT THE MOUTH -
Hobson, 29, told AFP he had been with the couple the day before Sturgess fell ill, and was with Rowley when he also collapsed several hours later.
"She was complaining of a headache in the morning and she went into the bathroom to have a bath and he heard a thump and she was in there having a fit, foam coming out of her mouth," he said.
Sturgess was taken to hospital but Hobson stayed with Rowley for several hours until he too began to complain of feeling ill.
"He was sweating loads, dribbling, and you couldn't speak to him," Hobson said.
"There was no response from him, he didn't even know I was there. It's like he was in another world, hallucinating."
Police initially assumed the pair had consumed contaminated drugs.
But samples from both patients were sent to Porton Down defence laboratory on Monday following concerns over their symptoms, police said.
Both remain in a critical condition and are at Salisbury District Hospital -- the same facility where the Skripals were treated.
The sites cordoned off include a park and a homeless hostel in Salisbury, as well as a pharmacy, church and the house in Amesbury.
Hobson said the man was a drug user and the woman had been living in the homeless shelter, which was evacuated on Thursday.
Skripal and his daughter Yulia were treated for weeks before being released from hospital.
The poisoning sparked a diplomatic crisis that saw Russia and the West expelling dozens of diplomats in tit-for-tat moves.
