WRAPUP 3-Ukraine raises flag on recaptured island, but loses key supporter in UK's Johnson

* Ukraine hoists flag on recaptured Black Sea island

* Russia carries out air strike on island

* UK PM and staunch Ukraine supporter Johnson resigns

* Russia shells and probes new territory it seeks

By Max Hunder and Simon Lewis

KYIV/BAKHMUT, Ukraine, July 7 (Reuters) - Ukrainian forces raised their national flag on a recaptured Black Sea island on Thursday in a defiant act against Moscow, but Kyiv lost a main international supporter after British Prime Minister Boris Johnson said he would step down.

Russian forces also shelled potential conquests in eastern Ukraine ahead of an expected new offensive.

Moscow did not conceal its delight at the political demise of Johnson, a leader whom it has long criticised for arming Kyiv so energetically.

"The moral of the story is: do not seek to destroy Russia," Russian Foreign Ministry spokeswoman Maria Zakharova said. "Russia cannot be destroyed. You can break your teeth on it - and then choke on them."

Johnson said Britain's support for Ukraine would continue regardless but his resignation comes at a time of domestic turmoil in some other European countries that support Kyiv amid doubts about their staying power for what has become a protracted conflict.

Mykhailo Podolyak, a Ukrainian presidential adviser, thanked Johnson, saying he had been someone who had "began to call a spade a spade from the beginning".

Moscow was fast to respond to Ukraine's defiant flag-raising ceremony on Snake Island, located about 140 km (90 miles) south of the Ukrainian port of Odesa.

Its warplanes struck the strategic island shortly afterwards and destroyed part of the Ukrainian detachment there, it said.

Russia abandoned the Snake Island at the end of June in what it said was a gesture of goodwill - a victory for Ukraine that Kyiv hoped could loosen Moscow's blockade of Ukrainian ports.

Images released by Ukraine on Thursday showed three Ukrainian soldiers raising the blue and yellow national flag on a patch of ground next to the remains of a flattened building.

Andriy Yermak, the Ukrainian president's chief of staff, suggested the moment was one that would be repeated across Ukraine in the coming months.

"The flag of Ukraine is on Snake Island. Ahead of us are many more such videos from Ukrainian cities that are currently under temporary occupation," he wrote on Telegram.

Russia's missile strike on the island's new residents had caused significant damage to its dock, Odesa regional administration spokesman Serhiy Bratchuk said.

In Moscow, the Russian defence ministry said several Ukrainian troops had landed on the island before dawn.