FRANKFURT (AFP) - Greece's Prime Minister Alexis Tsipras has warned German Chancellor Angela Merkel in a letter that Athens will not be able to service its debt without financial help from the European Union, the Financial Times reported on Monday.

The newspaper said it has a copy of a letter dated March 15 in which Mr Tsipras "warns that his government will be forced to choose between paying off loans, owed primarily to the International Monetary Fund, or continue social spending." "With this letter, I am urging you not to allow a small cash flow issue, and a certain 'institutional inertia', to not turn into a large problem for Greece and for Europe," Mr Tsipras wrote.

Mr Tsipras is to meet Dr Merkel in Berlin later on Monday. The Greek leader has blamed Dr Merkel's insistence on tough austerity for his country's "humanitarian crisis" of poverty and mass unemployment.

Dr Merkel, for her part, insists that if cash-strapped Greece wants more bailout loans, the biggest share of which is financed by Germany, it must accept the bitter medicine of spending cuts and reforms.

Greece's creditors agreed in February to extend its 240 billion euro bailout by four months in exchange for promises of further reforms.

At an EU summit last week, Greece lobbied Brussels to release vital funds to help it make payments to creditors in the coming days, and avoid bankruptcy and a possible exit from the euro.