"After four days of lengthy and detailed talks, reflecting the complexity of the technical issues discussed, it became clear that further work is needed," spokesman Michael Mann said.
"There will now be consultations in capitals, in the expectation that technical talks will continue soon," he said via email.
The statement came after Iranian state media reported that Tehran's negotiators had returned home after Washington blacklisted a dozen companies and individuals for evading US sanctions.
Under a deal struck in Geneva on November 24 world powers agreed not to impose fresh sanctions during a six-month roll-back of parts of Iran's nuclear programme.
Senior US administration officials argued however that Thursday's measures were taken as part of the existing sanctions regime.
The talks in Vienna, which began on Monday, involved experts from Iran and the P5+1 world powers -- the United States, China, Russia, Britain, France and Germany -- plus the International Atomic Energy Agency, which will verify Iran's freeze.
Once the starting gun fires on the six months, Iran and the P5+1 are due to negotiate a long-term "comprehensive" accord to end once for all the decade-old standoff over Tehran's nuclear programme.
