Ronald Zak, AP
U.S. Secretary of State John Kerry speaks to the media after closed-door nuclear talks on Iran in Vienna, Austria, Tuesday, July 15, 2014. Intense negotiations with Iran have yielded “tangible progress,” U.S. Secretary of State John Kerry said Tuesday, but significant gaps remain ahead of a July 20 target date for a deal meant to put firm curbs on Tehran’s nuclear activities in exchange for an end to sanctions. (AP Photo/Ronald Zak)

VIENNA — Intense negotiations with Iran have yielded "tangible progress," U.S. Secretary of State John Kerry said Tuesday, but significant gaps remain ahead of a July 20 target date for a deal meant to put firm curbs on Tehran's nuclear activities in exchange for an end to sanctions.

Kerry, who joined three other Western foreign ministers on the weekend to add diplomatic muscle to the talks said the negotiations would continue until at least Sunday. In the meanwhile, he said he would consult with President Barack Obama and U.S. Congress on where the talks were "as well as a path forward" if no pact is agreed on by July 20.

"There has been tangible progress on key issues," he told reporters. "However there are very real gaps on other key issues."

Kerry spoke after multiple conversations with Iranian Foreign Minister Mohammed Javad Zarif and separate meetings with the foreign ministers of Britain, France and Germany, who also converged on the Vienna talks in an attempt to move them forward.

An extension of up to six months is possible for the negotiations, by mutual agreement. But judging by Kerry's comments, any decision to go past Sunday will be taken in the closing hours leading up to that date.

The main dispute is over Iran's nuclear enrichment program. Tehran says it needs to expand enrichment to make reactor fuel but the U.S. fears Tehran could steer the activity toward manufacturing the core of nuclear missiles. The U.S. wants deep enrichment cuts; Iran wants to greatly expand enrichment.