The Ukrainian President used Twitter to deliver the shock warning on Saturday. He added the Russian government is seeking to reverse Ukrainian independence. It comes after Russian forces seized a number of Ukrainian ships and their crews following an altercation in the Sea of Azov.
In the tweets, Mr Poroshenko alleged a substantial buildup of Russian forces in the Crimea and Donbass regions of the country, in southern and eastern Ukraine respectively.
In total he stated there are now around 80,000 Russian troops, 900 tanks and 2,300 armoured vehicles within sovereign Ukrainian territory.
He added these are supported by 1,400 artillery pieces and missile systems, 500 aircraft and 300 helicopters.
Mr Poroshenko previously claimed to have evidence Russia is planning a ground invasion of Ukraine.
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READ MORE: Putin SHUNS international demands to release Ukrainian sailors
Last week, Russian forces seized three Ukrainian vessels as they attempted to pass by Crimea through the Kerch Strait.
During the attack shots were fired by the Russian ships and a number of Ukrainian sailers were wounded.
As many as 12 of the sailers are awaiting trial for allegedly violating the Russian border and have been taken into custody for two months.
In response Mr Poroshenko declared martial law in 10 Ukrainian regions which are close to Russia.
Speaking to reporters at the G20 summit in Argentina about the escalating conflict, Putin said: “The current Ukrainian authorities have no interest in resolving the conflict, especially by peaceful means.
“As long as they remain in power, the war will continue.”
Russian forces seized Crimea in 2014 after protestors forced Kremlin sympathetic Ukrainian president Viktor Yanukovych from power.
Pro-Russian separatists in Eastern Ukraine also seized land, and are widely believed to have had support from regular Russian forces.
They went on to create two currently unrecognised statelets, the People’s Republic of Donetsk and the People’s Republic of Luhansk.
Their actions sparked fierce fighting between separatists, backed by Russian troops, and the Ukrainian military.
Deadly flareups continue to occur regularly, though the level of violence is significantly below its peak in 2015 and 2016.
In September Defence Secretary Gavin Williamson announced the UK would send additional troops to Ukraine to help train the Ukrainian military.