Vienna, Austria (CNN) -- Negotiations between Western powers and Iran to reach a permanent agreement on the future of Tehran's nuclear ambitions are likely to be extended beyond the July 20 deadline, a senior U.S. official and a Western diplomat told CNN on Tuesday.
The sources, who did not speak for full attribution, said the United States and its partners in the negotiations -- Britain, France, Germany, China and Russia -- had not started talking about an extension with the Iranians.
But there are a number of potential time frames being discussed among the United States and its partners, ranging up to six months, the sources said.
Secretary of State John Kerry said on Tuesday in Vienna the negotiations have not yet bridged some "very real gaps."
Kerry told reporters there had been "tangible progress on key issues," but negotiators have yet to arrive at a "workable formula."
He said he would return to Washington to consult with the White House and lawmakers on options for moving forward, including whether to extend the talks beyond the deadline.
"Our team will continue working very hard to try to reach a comprehensive agreement that resolves the international community's concerns," Kerry said of the negotiators.
Despite the continued differences, Kerry said he believed there was a way ahead.
"But -- and this is a critical point -- while there is a path forward, Iran needs to choose to take it. And our goal now is to determine the precise contours of that path, and I believe we can," he said.
Responding to reporters' questions, he added that trust has to be built because of Iran's past track record in pursuing a secret nuclear program.
Tehran insists its nuclear ambitions are peaceful, but the world powers fear it plans to build nuclear weapons.
"We don't see any benefit in Iran developing a nuclear weapon," Iran's Foreign Minister Javad Zarif told NBC's "Meet The Press" on Sunday.
He said Iran has a number of advantages over its neighbors, including "the fact that we have better technology," which Iran doesn't need to augment with nuclear weapons.
While a six-month extension would give them the maximum negotiating time, the Obama administration has domestic considerations to weigh, including a November deadline after which new sanctions on Iran would have to be imposed under U.S. law.
There are also opponents in Congress who could use the chance for an extension to try to torpedo a deal.
When asked about prospects for an extension, White House spokesman Josh Earnest said he was "not in a position to speculate about which path will be taken."
An interim deal that took effect in January required Iran to dismantle or freeze some of its nuclear program and open it to more international inspections in return for limited relief from crippling international sanctions.
That set the stage for the current talks on a permanent accord.
CNN's Laura Smith-Spark contributed to this report. Labott reported from Washington and Crawford from Vienna.