Mykhailo Podolyak added that the mobilisation was a predictable step that would prove extremely unpopular in Russia, the Reuters news agency reported.
"Absolutely predictable appeal, which looks more like an attempt to justify their own failure," Podolyak said, as quoted by Reuters.
"The war is clearly not going according to Russia's scenario," he added.
Ukraine 'winning this war,' Russia 'becoming global pariah': UK's Wallace
Meanwhile, British Defence Secretary Ben Wallace was quoted as saying that the mobilisation was an admission by Putin that "his invasion is failing."
"President Putin's breaking of his own promises not to mobilise parts of his population and the illegal annexation of parts of Ukraine are an admission that his invasion is failing," Wallace said in a Twitter post.
He added that Putin "and his defence minister have sent tens of thousands of their own citizens to their deaths, ill equipped and badly led."
"No amount of threats and propaganda can hide the fact that Ukraine is winning this war, the international community are united and Russia is becoming a global pariah," Wallace also said.
The US ambassador to Ukraine, Bridget Brink, said the Kremlin had shown weakness by announcing the mobilisation of reservists and planning referendums in Russian-held areas of Ukraine, according to Reuters.
Mobilisation of 300,000 reservists
Putin on Wednesday ordered a partial mobilisation of 300,000 reservists for his country's military campaign in Ukraine, news agencies reported.
The move marks Russia's first such mobilisation since World War II and threatens to bring about a major escalation of the war in Ukraine, Reuters reported.
In a televised address to the Russian nation, Putin said on Wednesday, as quoted by Reuters: "If the territorial integrity of our country is threatened, we will use all available means to protect our people - this is not a bluff."
He warned that Russia had "lots of weapons" to respond to what he described as the West's "nuclear blackmail."
Defence Minister Sergei Shoigu said on Wednesday that 5,397 Russian soldiers had been killed since the start of the conflict, which is now in its seventh month.
Ukrainian military authorities, meanwhile, put Russia's military death toll in the country at around 55,110.
On Tuesday, pro-Russian regional leaders announced referendums for September 23-27 in Ukraine's Luhansk, Donetsk, Kherson and Zaporizhzhia provinces, which represent around 15 percent of Ukrainian territory, according to Reuters.
A senior US State Department official has warned that there will be “increased consequences” if Russia annexes parts of Ukraine.
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Source: IAR, PAP, Reuters, ukrinform.net/ukrinform.net