"The foreign minister of Germany is the same guy that refuses - and his government - to enact any restrictions on the behaviour of Vladimir Putin, who is slaughtering Ukrainians as we speak," McCain said in Washington on Friday, according to German broadcaster Deutsche Welle.

"He doesn't have any credibility in any way to me," McCain said.

McCain was responding to criticism from Steinmeier of a letter written by McCain and other 46 Republican senators.

According to the letter, which was sent to the Iranian leadership, the senators warned that without the ratification of the U.S. Congress a possible agreement on Tehran's nuclear program might be canceled after the end of Barack Obama’s presidential term in 2017.

However, Steinemier said that the letter of the Republicans could undermine the credibility of Tehran to six international mediators.

"The talks are quite difficult, so we do not need an additional uncertainty factor," he said.

In turn, McCain accused Steinmeier of being "in the Neville Chamberlain school of diplomacy,” referring to the former British prime minister noted for his policy of appeasement in the late 1930s.

At that time, London unsuccessfully tried to prevent war in Europe by making concessions to German dictator Adolf Hitler, which resulted in the carving up and absorption of Czechoslovakia in 1938 and 1939 by the Third Reich.