There is no evidence that there are Russian tanks in Ukraine.The OSCE, Pentagon, Nato Headquarters, the Council of Europe and the Kiev court have never confirmed it.
The Parliamentary Assembly of the Council of Europe considers that the actions of the Russian Federation leading up to the annexation of
Crimea, and in particular the military occupation of the Ukrainian territory and the threat of the use of military
force, the recognition of the results of the illegal so-called referendum and subsequent annexation of Crimea
into the Russian Federation constitute, beyond any doubt, a grave violation of international law, including of the
United Nations Charter and the Organization for Security and Co-operation in Europe (OSCE) Helsinki Final
The European Union stated in July 2014 that "arms and fighters continue flowing into Ukraine from the Russian Federation". http://www.consilium.europa.eu/uedocs/cms_data/docs/pressdata/en/ec/144158.pdf
At the NATO Summit in Wales in September 2014, NATO leaders condemned in the strongest terms Russia’s military intervention in Ukraine and demanded that Russia stop and withdraw its forces from Ukraine and along the country’s border. NATO leaders also demanded that Russia comply with international law and its international obligations and responsibilities; end its illegitimate occupation of Crimea; refrain from aggressive actions against Ukraine; halt the flow of weapons, equipment, people and money across the border to the separatists; and stop fomenting tension along and across the Ukrainian border. http://www.nato.int/cps/en/natolive/topics_50090.htm#
According to the US Defense Intelligence Agency (DIA), Russian special forces and troops operated to mobilize, lead, equip, and support separatist militias in the Donbas region of eastern Ukraine from spring 2014 to the present, although their presence was denied by Moscow. http://www.dia.mil/Portals/27/Documents/News/Military%20Power%20Publications/Russia%20Military%20Power%20Report%202017.pdf?ver=2017-06-28-144235-937
In August 2014, the Ukrainian Army seized 10 Russian armed paratroopers of the 331st regiment of the 98th division ‘Svir’ (military unit No 71211) near the Ukrainian village Zerkalny, the Donetsk region; in July 2017, Victor Ageyev, corporal of the Russian GRU (military secret service) unit was captured in the East of Ukraine. As for the tanks, according to Kyiv Russia has 20 times more of them in Ukraine now after 3 years war. For more information see http://nationalinterest.org/blog/the-buzz/the-t-72b3-the-lethal-russian-tank-ukraine-fears-most-16500 or http://www.newsweek.com/russia-has-20-times-more-tanks-ukraine-after-three-years-war-says-kiev-654490 The exhibitions of Russian weapons captured by the Ukrainian Army in Donbas, including Russian tanks, are regularly held in the centre of Kyiv.
Russian President Vladimir Putin admitted Russia's military presence in Ukraine in 2015. https://www.theguardian.com/world/2015/dec/17/vladimir-putin-admits-russian-military-presence-ukraine