"No doubt, the whole convoy approach serves the purpose, at least, to distract attention from possibly more overt military operations," Anders Fogh Rasmussen said in an interview with the Financial Times, Censor.NET reports.
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"The main reason I'm a bit more concerned now is that the Ukrainian military progress in the east might tempt the Russians to increase their support for the separatists in different ways."
He said evidence of increased Russian military intervention in the conflict included reports "from multiple sources" of Russian artillery firing from both inside Ukraine and from just over the border. He said Russia's build-up of forces along the border has also continued to increase.
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Mr Rasmussen said the Kremlin's failure to allow Ukrainian or Red Cross authorities to inspect a first convoy that crossed into Donbas on Friday raised questions of whether the trucks were really a resupply mission for Russian-backed separatists, saying similar questions exist about Mr Lavrov's new convoy.
"You can't exclude that so-called humanitarian operations are a cover for a military intervention," the Nato chief said, noting that Kyiv already supplies humanitarian aid to the region. "The major question remains whether this is really humanitarian aid or [if] it is a resupply of the armed separatists in eastern Ukraine. If there is nothing to hide, why not accept the International Red Cross?"
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