Vladimir Putin is testing the UK at what the Russian president regards as a time of weakness owing to Brexit, Lithuania’s foreign minister has told the Guardian.

Speaking after a meeting with the British foreign secretary, Boris Johnson, on Thursday, Linas Linkevičius said Russia saw the UK as increasingly isolated as it prepared to leave the EU. Russia was testing the UK’s strength, resolve and diplomatic links after the Salisbury nerve agent attack, he added.

“Russia is always looking for weak points, and may feel the UK does not feel very strong,” he said in an interview. “The Russian assumption may be that in the process of Brexit, the UK is weaker in terms of its isolation, and due to Brexit the EU will not be very enthusiastic in backing the UK up.

“Fortunately that is not the case, and we will support the UK, but Russia acts by testing for reactions.”

Linkevičius said the introduction of a British version of the US’s Magnitsky Act – one of the sanctions announced by Theresa May on Wednesday – would be very painful for the Russian elite, adding that the measure had already been introduced in Lithuania.

“You know how many oligarchs find safe haven in London – their money, the real estate, the children sent to secondary schools – and they cannot imagine their life without that,” he said. “Can you imagine if people were put on a UK list who already have real estates and property here? Of course it will be painful.”

Linkevičius said Putin’s actions represented a threat to liberal democracy. “They test and deny. I am not asking for escalation, but if no clear messages are sent, Russia regards it as an encouragement to do more.”

Lithuania, long on the frontline of perceived Russian aggression, is one of the UK’s closest allies in combating Moscow. Linkevičius said he expected EU foreign ministers to offer a full-throated defence of the UK at a meeting on Monday, at which Johnson will seek the bloc’s support.

In a joint statement issued on Thursday, France, Germany, the US and the UK said no country but Russia could plausibly be responsible for the attack on the former Russian double agent Sergei Skripal and his daughter in Salisbury, Wiltshire.

The countries said the nerve agent attack represented an assault on UK sovereignty, and demanded Russia provide full disclosure of its novichok nerve gas programme to the Organisation for the Prohibition of Chemical Weapons. Britain has alleged that novichok was used in the attack on Skripal; Russia denies being behind the poisoning, which also left a British police officer, Nick Bailey, seriously ill in hospital.

On Thursday, France hardened its support for the UK. The Élysée Palace issued a statement saying the UK assumption that the Russian state was responsible was the only plausible explanation for the attack.

The statement was issued after a phone call between May and the French president, Emmanuel Macron, which followed headlines in the British press saying France was undermining the UK by refusing to declare Russia responsible.

The French ambassador to the UN, François Delattre, told the UN security council on Wednesday that France had full confidence in the British investigation to quickly shed light on the precise circumstances of “this use of a chemical weapon”.

Linkevičius said he trusted the British explanation of the source of the attack, adding: “If we have a choice, we trust our allies. It is very hard to trust Russia. Russia denies what it is doing in Syria, that it has any presence in Ukraine, and before the annexation of Crimea they denied everything.”

He said the Salisbury poisoning “was a message to future double agents that they will not be safe wherever they go in the world, and not just them, but also their families.

“Unfortunately, Russian tactics are to test to see what the reaction is, and if the response is weak they continue in the same spirit. After the invasion of Georgia in 2008 there was condemnation, but very soon it was business as usual. That was a lesson learned by them, but not a lesson learned by us.”

Linkevičius called for the removal of any loopholes in the chemical weapons treaty, greater pooling of EU resources on cybersecurity – including a rapid-reaction team – and a shift away from EU dependence on Russian gas.

He said many of Europe’s northern littoral states disagreed with Germany, which continues to argue that the proposed Nord Stream 2 gas pipeline from Russia to Germany over the Baltic Sea was a purely commercial decision. “It is very dangerous to give Russia a monopoly. When you have a single source for energy, you are very vulnerable. In the Moscow military parade a Gazprom pipe was on display. It was on display for a reason.”

He also called for Nato air defence to be stepped up in the Baltic states, saying the current presence was not sufficient given the build-up of Russian weapons.

Linkevičius urged the EU not to relax economic or individual sanctions against Russia over its military intervention in Ukraine. “Russia denies there is any impact, but it is very painful. They are trying to divide us and so we must be cohesive.”