MOSCOW - Russian Foreign Minister Sergei Lavrov on Monday suggested the British government could be behind the poisoning of a former double agent that has triggered an unprecedented wave of tit-for-tat expulsions of diplomats.

More than 150 Russian diplomats have been ordered out of the US, EU members, NATO countries and other nations as punishment for the March attack on Sergei Skripal and his daughter Yulia in the English city of Salisbury which Britain has blamed on Moscow.

The counterclaims from Moscow came as the last Russian diplomats were set to leave the countries where they were posted. A Russian diplomat left Macedonia along with family members earlier Monday and was seen off at the airport by the Russian ambassador, TASS state news agency reported.

The Kremlin has vehemently denied the accusations that it was behind the nerve agent attack and ordered Britain and its allies to recall some of their envoys from Russia.

On Monday, Moscow’s top diplomat Lavrov told a news conference that the Skripal attack “could be in the interests of the British government which found itself in an uncomfortable situation having failed to fulfil promises to its electorate about the conditions for Brexit.”

He argued that Moscow had no reason to poison Skripal - whom it convicted of treason and handed over in a 2010 spy swap - on the eve of a presidential election in March and months before Russia hosts the World Cup.

Lavrov suggested that the poisoning of the Skripals “could also be in the interests of the British special forces who are known for their abilities to act with a licence to kill”. At the same time, he denied the attack’s “sophistication” meant it was likely to have been approved by the Russian leadership, as CNN cited a source briefed on the investigation as saying. “If I understand correctly, sophisticated attacks usually lead to instant death,” Lavrov said. Britain has called it “highly likely” that Russia was responsible for the attack using a nerve agent developed in the USSR. Lavrov also accused Britain and the US and its allies of “casting off all decency” and “resorting to lies and disinformation.”

He insisted that “serious experts and “leaders of a whole number of countries” were questioning Britain’s account of the crime. “How far we all go” did not depend on Russia, he said, adding Moscow would follow “the principle of reciprocity.”

Britain has dismissed the various, often contradictory, theories, that Russia has put forward, with even the Foreign Office publicly mocking them on social media. Lavrov complained that the British authorities have still not given consular access to “our citizens,” and that the situation had not changed despite Yulia Skripal’s improved condition. He said he hoped Sergei Skripal would also get better.

Moscow has sought to punish Britain for calling on allies to expel Russian diplomats, saying Saturday London must reduce its diplomatic presence in the country by more than 50 more diplomats, to achieve “parity” with Russia’s diplomatic staff in Britain.

Russia had earlier expelled 23 British diplomats and last week told envoys from 23 other countries that some of their staff must go home.

Separately, a top Kremlin aide said Monday that US President Donald Trump proposed a White House summit when he called President Vladimir Putin last month, prior to the expulsion of Russian diplomats from the US.

In Washington, officials dismissed the statement from Putin’s top foreign policy aide, Yury Ushakov.

A senior US administration official said “it’s obviously in their interest” to publicise the summit invitation as Putin’s risks deeper international isolation after the Skripal attack.

White House Press Secretary Sarah Sanders said the two leaders had discussed a bilateral meeting “at a number of potential venues, including the White House.”

On March 20, Trump called Putin to congratulate him on his re-election, and the US leader told reporters afterwards that the two would “probably get together in the not-too-distant future.”

Both the US official and Ushakov said that planning for the meeting had not yet begun.

Ushakov expressed the hope that Russia and the United States could return to “constructive and serious dialogue.”

After the phone call, Washington expelled 60 Russian diplomats and shut down a Russian consulate in Seattle. Moscow responded by sending home 60 US diplomats and closing Washington’s consulate in Saint Petersburg.

Washington has, however, said Russia was free to apply to accredit more diplomats to replace those expelled.