The European Union has decided keep in place biting economic sanctions on Russia over its alleged backing of pro-Kremlin insurgents who have largely ignored a four-week Ukrainian truce.

The bloc’s announcement on Tuesday came just moments before NATO reported the continued presence of “hundreds” of elite Russian soldiers in the war-torn former Soviet nation’s separatist east.

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The two Western alliances’ continuing pressure on Russia – already more isolated than at any point in Vladimir Putin’s 15-year rule – set an ominous tone to the launch of Ukraine’s October 26 parliamentary election campaign.

EU spokeswoman Maja Kojicancic said that while member states saw “encouraging developments” in parts of the September 5 truce agreed by Moscow and Kiev, other parts “need to be properly implemented” before sanctions could be eased.

The Ukraine truce was reinforced with a September 20 deal under which each side agreed to create a buffer zone by pulling back its forces 15km from the front line.

European monitors were also to be permitted to report on any resumption of bloodshed in the conflict, which has already claimed more than 3200 lives.

But a weekend attempt by a Russian military delegation to convince the rebels to comply with the truce ended in seeming failure – though the visit itself represented an indirect admission by Moscow that it did indeed have influence over the insurgency.

The Ukrainian military said militias on Monday launched a tank assault on a long-disputed airport outside the rebel stronghold of Donetsk in which a shell hit an armoured vehicle filled with government troops.

At least nine soldiers died in the strike and ensuing firefight, while officials reported the death of five civilians by Tuesday evening.

The joint US and EU sanctions on Russia’s largest banks and energy companies aim to cut off the Kremlin from its main sources of income and erode Putin’s domestic support.

But the economic struggles have done little to slow Putin’s seeming – but denied – ambition of keeping a permanent hold on Ukraine’s Russian-speaking east and undermining Kiev’s historic shift toward the West.

NATO on Tuesday said “hundreds of Russian troops, including special forces, still remain inside Ukraine” despite a “significant” recent withdrawal.

Moscow argues that any Russian soldiers in Ukraine went there while off duty and based on heartfelt political convictions rather than any orders from the Kremlin.