Molyviatis cancelled his trip to the EU summit in Brussels on news the US is considering giving official recognition to Macedonia.
The Greek foreign minister summoned US ambassador to Greece Thomas Miller to clarify whether the US was set to recognise the government in Skopje as Macedonia, foreign ministry spokesman Georges Koumoutsakos said in a statement on Thursday.
Greece objects to the fact that the former Yugoslav republic, which gained independence in 1991, took the name of Macedonia.
The name is also that of a northern Greek province and viewed by Athens as part of Greek heritage.
The Greek government has insisted that Macedonia be known officially as the Former Yugoslav Republic of Macedonia, or FYROM for short.
It was under that name that the new country was admitted to the United Nations in 1993.
Macedonian officials confirmed on Thursday that the US officially recognised their country as the Republic of Macedonia.