A defiant Alexis Tsipras -- leader of Syriza, Greece's coalition of radical left-wing parties -- said Europe would be foolish to cut Athens off from funding, but if it did, the country would simply default on its debts.
His comments are set to raise the stakes in an already tense standoff between Greek politicians and their European benefactors.
Tsipras, 37, is head of the Coalition of the Radical Left party, known as Syriza, and he had been leading in the polls ahead of fresh elections in June.
Our first choice is to convince our European partners that, in their own interest, financing must not be stopped, Tsipras told the Wall Street Journal.
If we can't convince them -- because we don't have the intention to take unilateral action -- but if they proceed with unilateral action on their side, in other words, [if] they cut off our funding, then we will be forced to stop paying our creditors, to go to a suspension in payments to our creditors, he said.
On Wednesday, however, Greece's center-right New Democracy party gained a slight edge among voters over Syriza, ahead of the June 17 parliamentary elections.
New Democracy led Syriza 23.1 percent to 21 percent, according to Alpha TV's survey of 1,027 Greeks.
June's elections will be a repeat of parliamentary elections held earlier this month, and they were called by President Karolos Papoulias after parties failed to form a coalition government. Syriza finished a surprising second place in the May 6 elections.
Whatever we do, things will be difficult, Tsipras told the Journal.
But it will also be difficult at the same time for all of Europe because the euro will collapse, if Greece's funding is cut off, he said.
Elsewhere, Fitch ratings downgraded Greece a further two notches into 'junk' status on Thursday, as fears of a default grew about the country leaving the euro zone.