America's secretary of state Mike Pompeo, had assured him in a phone call that his country, had the "full support, full assistance, including military assistance, full coordination, what we [need] to do to protect Ukrainian sovereignty and territorial integrity”, Mr Poroshenko said.
Addressing a suggestion that Donald Trump had been slow to back Ukraine over the stand-off, the Ukrainian leader told CNN host Christiane Amanpour, that the president “in his speech, also supported Ukrainian territorial integrity and [has] been on our side”
The US president had earlier said: “We do not like what’s happening either way. We don’t like what’s happening, and hopefully it will get straightened out.”
The Independent has contacted the State Department for comment.
Mr Poroshenko’s comments came after Russia seized three Ukrainian navy vessels it accused of having ventured into its territory over the weekend.
Vessels controlled by Kremlin spy agency, the FSB, fired on the Ukrainian boats, injuring six crew members before they were seized.
Kiev and Moscow have each blamed the other for the incident in the Kerch Strait, which links the Black Sea and Azov Sea near the Crimean peninsula, which Russia annexed in 2014.
Moscow has been building up infrastructure in the region since then.
Russian president Vladimir Putin told German chancellor Angela Merkel by phone that his country was ready to provide more details to bolster its version of events. Moscow says Kiev deliberately provoked it in order to trigger a crisis.
Ms Merkel, who later spoke with Mr Poroshenko, called for de-escalation and dialogue.
But Mr Poroshenko told CNN that Mr Putin would not speak to him directly.
Ukraine has introduced martial law for 30 days in parts of the country it deems most vulnerable to an attack from Russia. It has said its ships did nothing wrong and that it wants the West to impose new sanctions on Moscow.
Russia's presidential spokesman, Dmitry Peskov, criticised the move, saying: "The introduction of martial law could potentially raise the threat of escalation of tensions in the region of conflict."
The FSB released video footage of the captured sailors saying they had ignored Russian orders to stop. At least one appeared to be reading from a script, while Ukrainian politicians said the sailors were coerced, rendering their confessions meaningless.
The security service said it had information showing the sailors’ mission had been pre-planned by the Ukrainian government and that two intelligence officers from Ukraine’s SBU security service had been on board to coordinate the provocation.
Russia has already started prosecuting the crew and a court ordered they be held for two months.
Additional reporting by agencies