The parties in Turkey discussed Iran's nuclear program for the first time in more than a year and agreed to reconvene in Baghdad on May 23.
Ben Rhodes, deputy national security adviser, said the United States sees room to negotiate over how Iran can meet international obligations under its nuclear program, which Tehran says is for energy and medical purposes but global powers fear is meant to create a weapon.
"We believe ... the talks in İstanbul have been a positive first step, that there was a constructive atmosphere, that the Iranians came to the table and engaged in a discussion about their nuclear program," Rhodes told reporters in Colombia, where US President Barack Obama is attending a regional summit.
The agreement to meet again in Baghdad next month was "an additional positive sign," he said.
Other countries involved in the talks also referred to them as "constructive."
The six powers included the permanent members of the UN Security Council - Russia, the United States, China, France and Britain - along with Germany, a group known as the P5+1.
Over the past year, Israeli and US warnings of military strikes if Iran does not stop working on some aspects of nuclear technology have stoked fear of war, and raised oil prices, in an unsettled Middle East.