Ukraine’s foreign minister threatened Russia with severing diplomatic relations on Sunday as scores of people were reported killed in days of Ukrainian government airstrikes near the eastern city of Kramatorsk.

Ukraine also observed a day of mourning for the 49 personnel killed on board a military aircraft that was shot down Saturday by separatists near Luhansk, another city in the troubled east.

Flags flew at half-staff and all entertainment and sporting events were cancelled after President Petro Poroshenko threatened the pro-Russian separatists with retaliation.

A minute of silence was observed at noon, and prayers were said in churches for the victims of the deadliest attack on Ukrainian forces since they began what the central government calls an “anti—terrorism offensive” in mid-April.

More than 50 separatists were killed and about 150 injured in the airstrikes near Kramatorsk, military spokesman Vladislav Seleznyov said.

Many of the casualties were from the neighbouring restive region of the North Caucasus, he said.

Separatist leader Valery Bolotov said 100 fighters and civilians had been killed.

On Saturday, Mr Poroshenko warned the separatists they could expect a response to the downing of the air force Ilyushin-76, saying, “All those who are involved in the cynical terrorist attack of this scale must be punished. ... Terrorists will receive an adequate response.” Eastern Ukraine has seen weeks of clashes between security forces of the central government in Kiev and the separatists.

The government in Kiev has accused Russia of supporting the separatists, which Moscow has denied. The latest salvo in their diplomatic battle came from Ukrainian Foreign Minister Andriy Deshchytsia, who said if Moscow continues to aggravate the unrest in eastern Ukraine, Ukraine would have to resort to the “extreme measure” of cutting diplomatic ties.

He accused Russia of failing to stop supplies for separatists from crossing their common border and said the security council in Kiev on Monday would discuss possibly closing the border.

Meanwhile, Russia and Ukraine were to resume talks late Sunday in Kiev on their gas—pricing dispute.

Russia has given cash—strapped Ukraine until 0600 GMT Monday to settle about 2 billion dollars in gas arrears or face the prospect of being delivered gas only after prepayment.

Energy ministers from Russia and Ukraine have been engaged in EU—mediated talks since April to try to settle their differences.

Ukraine is a key transit route for Russian energy flows to Western Europe and any halt in supplies would affect several EU member states dependent on Russian gas.

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