Interior Ministry says the accused recently returned to Spain after travelling to Ukraine in 2014.
David Cameron made the announcement this afternoon.
The ceasefire agreed in Minsk last week is now on the brink of total collapse as the rebels advance on Mariupol.
Poroshenko says that he believes this will be most efficient guarantor of security.
The peace deal, which only came into force this weekend, already looks under threat.
The town of Debaltseve came under heavy shelling despite the ceasefire being announced.
Russia reinforced rebels with 110 armored vehicles during marathon peace talks in Minsk, a spokesman said.
The sides have held only one meeting since agreeing to a ceasefire in September.
The separatist leader in Donetsk ruled out a ceasefire fighting intensified.
More than 4,700 have died since the conflict broke out nine months ago.
"So far a preliminary agreement has been reached that a meeting in Minsk will take place on the ninth of December."
New EU data shows the economic costs for Europe of pressuring the Kremlin
Ukraine said on Wednesday it was redeploying troops in the east because of fears separatists will launch a new military offensive.
A reporter saw a 50-vehicle column traveling toward the rebel stronghold of Donetsk in eastern Ukraine on Tuesday armed with rocket launchers and artillery guns.
A ceasefire to end a war that has killed 4,000 people has appeared shakier than ever, with both sides accusing the other of having violated the peace plan.
"The world is on the brink of a new Cold War. Some say that it has already begun," said the former leader of the Soviet Union
A security official warned that separatists would try and 'discredit' Kiev ahead of elections on Sunday amid uncertainty over how the vote will be conducted
Ukraine's warring sides agreed on Friday to withdraw artillery to the outer limits of a buffer zone
Residents and combatants in eastern Ukraine welcomed the respite in a five-month conflict that has killed at least 2,600 people
Fighting continues in eastern Ukraine as a cease-fire deal is reached
Putin's undeclared war with Ukraine is placing an unbearable burden on the families of Russian soldiers
Ukraine's president claims that direct Russian intervention in the conflict has tipped the balance in favour of separatist forces
EU leaders threatened fresh sanctions against Russia in response to ongoing aggressive military action in eastern Ukraine
Kiev government asks to join NATO as Putin directs separatist rebels
Pictures of Russian military activity contradicts Putin's denial that he ordered invasion
President Poroshenko claims Russia has escalated the military conflict, sending equipment and troops to southeastern Ukraine
Putin and Poroshenko meet as Russian troops are taken prisoner on Ukraine soil
Ukrainian and Russian forces engage in intense fighting one day ahead of peace talks