Greece would struggle to find creditors outside the EU and IMF, German finance minister Wolfgang Schaeuble has said.
He said it would be welcome to try to find investment from Beijing or Moscow, but may have difficulties.
His warning came after fears of a Greek debt default saw its borrowing costs jump 3.5 percentage points to 27%.
Greek Finance Minister Yanis Varoufakis said his government refuses to consider leaving the EU: “Toying with Grexit… is profoundly anti-European.”
He also promised to “compromise, compromise, compromise without being compromised” to satisfy current creditors.
Both men were speaking at talks in Washington.
On Wednesday, ratings agency S&P downgraded Greece’s credit rating.
Yields also rose on longer-term Greek borrowing, with the 10-year bond yield – the amount investors demand for lending – rising one percentage point to 13%.
Mr Schaeuble said that the Greek government needs to find creditors.
“The Europeans have said, OK, we are ready to do it [lend money] until 2020… If you find someone else, whether it’s in Beijing, in Moscow, in Washington DC, or in New York who will lend you money, ok, fine, we would be happy. But it’s difficult to find someone who is lending you in this situation amounts [of] €200bn.”
He added that Greece must focus on increasing its competitiveness and primary surplus.
Mr Schaeuble was speaking after the Greek government’s borrowing costs surged on Thursday.
The Financial Times had earlier reported that Greece had made an “informal approach” to the International Monetary Fund to have its bailout repayments delayed, but had been rebuffed.
But the head of the International Monetary Fund (IMF), Christine Lagarde, said at the World Bank spring meeting in Washington: “We have never had an advanced economy asking for payment delays.
“Payment delays are analysed as additional financing granted to that country. Additional financing means additional contribution by the international community – some of which are in much direr situations than the country eventually seeking those delays.
“Payment delays had not been granted by the board of the IMF in the last 30 years and it was eventually granted to a couple of developing countries and that delay was not followed by very productive results.
“It’s clearly not a course of action that would actually fit or be recommendable in the current situation.”
Greece owes the IMF some €1bn (£720m, $1.06bn) in repayments next month.
Many in the markets think the Greek government will struggle to make those payments if it does not agree an economic reform package with European creditors soon.
Failure to agree a plan with creditors will mean that the country will default, a development that could force the government to put limits on money transfers and even lead the country to leave the euro.
EU spokesman Margaritis Schinas said on Thursday that the EU was “not satisfied with the level of progress made so far” in debt negotiations.
BBC economics editor Robert Peston said if Greece misses a debt payment, that does not necessarily mean it will leave the euro.
“The government could follow the example of Cyprus and impose restrictions on the export of capital from the country, to conserve as much cash as possible in a banking system too close to collapse for comfort,” he said.
“And it could create its own IOUs, a sort of parallel domestic currency interchangeable with euros, to pay its employees and trade creditors.”
Mr Schaeuble had warned that he did not expect an agreement between Athens and its creditors in the next week.
But Greek Prime Minister Alexis Tsipras on Thursday said he was “firmly optimistic” the Greek government could reach a deal with its creditors.
“Despite the cacophony and erratic leaks and statements in recent days from the other side, I remain firmly optimistic that there will be an agreement by the end of the month,” Mr Tsipras said.
According to Mr Tsipras, several points of agreement had been found since talks first started, including on areas such as tax collection, corruption and initiatives to distribute the tax burden on those who have the ability to pay.
But he said the two sides still disagreed on four areas: labour issues, pension reform, an increase in value-added taxes and privatisations, which he referred to as “development of state property”.
In a later tweet, he said he was “certain that Europe will choose the path to democracy”.