Church bells rang in Kiev's Maidan square and hundreds of mourners bowed their heads as Ukraine remembered the 49 soldiers killed when their transport plane was shot down by pro-Russian separatists.

Flags were also lowered across the nation on Sunday, a day after Ukrainian forces suffered their single-biggest loss of life in the two-month push to reclaim control of separatist areas in the industrial east.

Ukraine's president, Petro Poroshenko, promised a tough response against the rebels who shot the plane down as it travelled to Luhansk.

But some mourners gathered in Kiev said the operation against the separatists should be stopped.

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Al Jazeera's Kim Vinnell, reporting from the eastern city of Donetsk, said operations against the separatist fighters were continuing.

Rebels said scores of their fighters were killed across the eastern region, but Al Jazeera could not independently confirm the claim.  

The downing of the military plane has increased tension as Moscow and Kiev try to agree how much Ukraine should pay for Russian gas. Kiev must pay $1.95bn in debts by Monday or have its gas cut off. Such a move could disrupt flows to the rest of Europe.

There was also a violent protest at the Russian embassy in Kiev on Saturday, and a diplomatic argument over comments by Ukraine's foreign minister about the president, Vladimir Putin.

On Sunday, Russia accused Ukraine's acting foreign minister, Andriy Deshchytsia, of "going beyond the limits of decency" by calling the Russian president a "prickhead" during the protest.  It demanded Deshchytsia's resignation.

Source: Al Jazeera and agencies