Sharjah24 - AFP: Ukraine warned Monday that Russian forces were preparing to intensify their fight for key cities in the Donbas, as President Volodymyr Zelensky bitterly accused Canada of undermining sanctions against Moscow.

In eastern Ukraine -- the focal point for a grinding Russian offensive -- the death toll from a weekend shelling of an apartment building in the town of Chasiv Yar in the Donetsk region rose to 33, according to emergency services.

Dozens of rescuers could be seen working amid the ruins of the partially destroyed building Monday, aided by a mechanical digger.

Earlier in the day deadly Russian rocket strikes slammed Kharkiv, the country's second city, in a targeted attack on "a shopping centre and civilian residences", regional chief Oleg Synyegubov said.

Prosecutors in his region said "six civilians were killed, including a 17-year-old and his father, who were driving past" at the time of the attack, according to the Interfax-Ukraine news agency.

The Ukrainian army warned, meanwhile, that Russian troops were likely planning to launch some of their heaviest attacks yet in the Donetsk region.

"There are signs of enemy units preparing to intensify combat operations in the direction of Kramatorsk and Bakhmut," it said, referring to two main cities still under Ukrainian control.

The latest attacks in Ukraine came as Europe braced for deeper cuts in gas supplies from Russia.

On Monday Russian gas giant Gazprom began more than a week of routine maintenance on its Nord Stream 1 pipeline -- with Germany and other European countries watching anxiously to see if the gas comes back on.

Over the weekend, Ottawa agreed to deliver to Germany a turbine that was undergoing maintenance in Canada, whose absence Russia had blamed for a decrease in supplies.

The move by Canada, which came despite sanctions in place against Russia and appeals from Ukraine, prompted Kyiv to summon Ottawa's ambassador.

"The Ministry of Foreign Affairs had to summon Canada's envoy to our country due to an absolutely unacceptable exception to the sanctions regime against Russia," Zelensky said in his daily address on Telegram.

Germany and other European countries are casting a worried eye on energy supplies after Italy's Eni and Austria's OMV said Gazprom was further reducing the supply of gas.

After Russia's invasion of Ukraine in February, Germany suspended certification of a second pipeline, Nord Stream 2, as fears grew over Europe's massive dependence on Russian gas.