LONDON, Sep 12 (Reuters) - Russian nationalists called angrily on Sunday (Sep 11) for President Vladimir Putin to make immediate changes to ensure ultimate victory in the Ukraine war, a day after Moscow was forced to abandon its main bastion in northeastern Ukraine.

The swift fall of Izium in Kharkiv province was Russia's worst military defeat since its troops were forced back from the Ukrainian capital Kyiv in March.

As Russian forces abandoned town after town on Saturday, Putin was opening Europe's largest ferris wheel in a Moscow park, while fireworks lit up the sky over Red Square to celebrate the city's founding in 1147.

In an 11-minute-long voice message posted to the Telegram messaging app, Chechen leader Ramzan Kadyrov, a Putin ally whose troops have been at the forefront of the campaign in Ukraine, dismissed the loss of Izium, a critical supply hub.

But he conceded the campaign was not going to plan.

"If today or tomorrow changes are not made in the conduct of the special military operation, I will be forced to go to the country's leadership to explain to them the situation on the ground," said Kadyrov.

Moscow's almost total silence on the defeat - or any explanation for what had taken place in northeastern Ukraine - provoked significant anger among some pro-war commentators and Russian nationalists on social media.

As the defeats unfolded, the Russian defence ministry on Friday posted video footage of what it said were troops being sent to the Kharkiv region.

On Sunday the defence ministry said Russian forces had struck Ukrainian positions in the region with airborne troops, missiles and artillery.