A day after Greece appeared on a collision course with its creditors, new radical left Prime Minister Alexis Tsipras has tamped down the rhetoric by vowing to pay off debts and not act unilaterally.

Finance Minister Yanis Varoufakis, who had a tense meeting with Eurogroup leader Jeroen Dijsselbloem in Athens on Friday, has brought forward a trip to Paris, London and Rome to meet his counterparts.

Tsipras said he never intended to act unilaterally, and expressed his certainty that Greece and the creditors will reach an agreement. He also pledged to pay back Greece's debt to the European Central Bank and the International Monetary Fund, which, along with the European Commission, form the "troika" of Greece's creditors.

U.S. Secretary of State John Kerry spoke to Greek Foreign Minister Nikos Kotzias by telephone on Friday to congratulate him on his appointment and underscore "the United States's interest in continued close bilateral co-operation with Greece," a senior State Department official said, speaking on condition of anonymity.

The official said they also discussed the situation in Eastern Ukraine and countering terrorism threats in the region.