Russia on Monday invited Greece to become the sixth member of the new regional bank for the BRICS countries that include Brazil, Russia, India, China and South Africa. The invitation was extended during a phone conversation between Greek Prime Minister Alexis Tsipras and Russia's Deputy Finance Minister Sergei Storchak, Agence France-Presse reported, citing a government source.

Tsipras, who reportedly called the invitation a “pleasant surprise,” said Athens would consider the option and formally discuss it with other officials of the BRICS countries at an upcoming economic summit in Russia in June, according to the source.

The Shanghai-based New Development Bank, formerly the BRICS Bank, was announced last June as a way of financing major infrastructure projects in the developing world. It is also widely seen as a counterweight to other major global finance institutions, including the International Monetary Fund (IMF) and the World Bank.

The latest move comes as Greece’s ruling left-wing Syriza party announced Sunday that it would make tougher demands upon the country's European creditors. "We have agreed on a tougher strategy to stop making compromises. We were unified and we have a spring our step once again," a Greek financial official said, according to the Telegraph.

Greece’s creditors from the eurozone and the IMF had demanded austerity reforms, including pension cuts, privatizations and an end to collective bargaining, as necessary conditions for Greece to gain access to bailout funds.

On Monday, Greece made another $834 million payment in debt interest to the IMF one day ahead of schedule.

Greek Finance Minister Yanis Varoufakis also warned that the country’s financial crisis would adversely worsen within weeks unless it secured more funds.

"The liquidity issue is a terribly urgent issue. It's common knowledge, let's not beat around the bush," Varoufakis told reporters in Brussels on Monday, BBC reported.