Top officials from 46 countries and blocs are taking part in a summit to discuss the issue of Russia's Crimea's annexation in 2014.
BBC, which reported the incident, says the documents found in Kent were from Britain's defence ministry and contain details about a British warship and Russia's potential reaction to its passage through the Black Sea.
From military to geopolitics, Ukraine would deliver NATO significant leverage over Russian influence in Eastern Europe and the Black Sea.
With Russia actively undermining the interests of NATO and its partners in the Black Sea, the Alliance must adopt a formidable strategy to enhance its presence in the region.
Ukraine says withdrawal of Russian military forces from the border could ease tensions but that the step alone would not stop the escalation or conflict in the eastern Donbass region.
The announcement came as President Vladimir Putin invited Ukrainian leader Volodymyr Zelenskyy for talks in Moscow following weeks of renewed fighting in the east of Ukraine between government troops and pro-Moscow separatists.
The exercise comes amid increasing violations of a ceasefire in Ukraine's east and a massive Russian troops buildup on the border with Ukraine that raised Western concerns.
Moscow has stationed thousands of troops on Ukraine's border and in annexed Crimea, EU says, with its top diplomat Josep Borrell warning "the risk of further escalation is evident."
Biden has described the sanctions, which are aimed at the Russian economy and dozens of individuals, as a "measured and proportionate" response to Moscow's hostile actions against the United States.
In a phone call with his Russian counterpart, US President Biden proposes a summit meeting in a third country in the coming months to discuss the full range of issues facing the United States and Russia, the White House says.
US Secretary of State Antony Blinken warns of “consequences” if Russia acts “aggressively” towards Ukraine amid rising alarm over Russian troop build-up on the border of the former Soviet republic.
Russia's deputy foreign minister raised concerns over what he said was increasing Black Sea naval activity by powers that did not have a coastline in the region, an apparent reference to the United States.
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