Greece is unable to repay the next loan installment to the International Monetary Fund due on June 5 without international aid, Nikos Filis, spokesman for the ruling Syriza party in Parliament, said Wednesday.

"Negotiations are tense. Now is the moment of truth," Filis told a local TV channel in Athens, as talks between representatives of the Greek government and the country's global creditors continued in Brussels on further arrangements regarding the Greek debt issue.

Athens hopes to strike a deal on post-bailout collaboration with lenders before the end of a scheduled EU summit in Latvia on Thursday, but creditors appeared skeptical.

At the moment there were "no funds in state coffers to repay the stupendous IMF installment" in two weeks, said Filis. "We cannot make it without a deal by then."

Greek Prime Minister Alexis Tsipras, Finance Minister Yanis Varoufakis and other Cabinet ministers said in recent weeks that the debt-laden country faces mounting liquidity issues. They assured, however, that pensions and salaries to public servants would be paid as usual.