Greece was trying to bridge major differences with creditors Wednesday on a plan to make more reforms in the country in exchange for bailout loans it needs to avoid a potentially disastrous default next week.

Leftwing Prime Minister Alexis Tsipras criticized the International Monetary Fund as being needlessly picky about the reforms Greece had proposed, which consisted largely of tax increases.

A Greek official confirmed the IMF was focusing on toning down the tax increases, saying they can hurt businesses. The official, who asked not to be identified because the talks were ongoing, said creditors are demanding, among other things, a freeze on pensions, scrapping some proposed taxes and surcharges on business, and higher sales tax on some goods.

"These are very tough negotiations," the Greek official said. "But there is a common will to get somewhere."

Tsipras said that as long as Athens delivered the right amount of savings, the IMF should have no say in what specific policies a sovereign country adopts.

"This odd stance seems to indicate that either there is no interest in an agreement or that special interests are being backed," Tspiras wrote on his official Twitter account.

The latest disagreement weighed on the Athens Stock Exchange, which dropped 1.8 percent after huge gains the previous two days. Government bond yields in Greece, Spain and Portugal rose, an indication of investor concern....