U.S. Secretary of State John Kerry is expected to meet his Iranian counterpart Javad Zarif for a second day in a row on Monday to see if they can make progress on reaching a deal on Tehran's nuclear program, a U.S. official said.
Kerry would "gauge Iran's willingness to make the critical choices it needs to make," the senior State Department official told Reuters.
The meeting comes as nuclear talks between Iran and six world powers appear likely to be extended past their target end date of July 20 after the sides failed to reach an agreement in Vienna on Sunday.
On Sunday, Kerry said major differences persist between Iran and the six world powers negotiating on Tehran's nuclear program. The remarks were echoed by Tehran and officials with knowledge of the talks said deep differences continued to separate the two sides.
Frank Walter Steinmeier, German foreign minister, said the ball was in "Iran's court now".
"It is now up to Iran to decide whether they are looking for a way to cooperate with the international community or if they want to remain in isolation," Steinmeier said.
"I hope the days until July 20th will be allow enough to reflect in Tehran and at the end pave the way for an agreement. The ball is in Iran's court now."
The main focus on Sunday was easing the dispute over Iran's enrichment program.
An extension of the talks would give more time to negotiate a deal that would limit the scope of such program in exchange for a full lifting of nuclear-related sanctions imposed on Iran.
Al Jazeera's James Bays, reporting from Vienna, said there had been some progress made since an interim deal was made in November, despite the parties' differences.
"Both sides are saying they'd rather get a deal now and the Iranian deputy foreign minister says that if there was an extension he only wants a matter of days," he said.
Araqchi said that he was "not pessimistic but also not very optimistic" about prospects for an agreement ahead of the self-imposed deadline. "No proposal has been accepted yet. We have not reached any agreement over the enrichment (program of Iran) and its capacity."
He added that if the talks collapsed, Iran would resume higher-level enrichment that it suspended on Jan. 20 when a preliminary accord the sides struck two months before took effect. Iran won limited relief from sanctions in return.
Iran says it is refining uranium to low levels of fissile purity to fuel a planned network of nuclear power stations. It earlier described its higher-level, or 20 percent purity, enrichment as material to fuel a medical research reactor. High-enriched uranium, or 90 percent, is for nuclear weapons.
Given the chasm in negotiating positions, some diplomats and experts believe the negotiations may need to be extended.
The Nov. 24 deal included a provision for lengthening talks on a permanent agreement by up to six months if all sides agree.
"There is a possibility of extending the talks for a few days or a few weeks if progress is made," Araqchi said.
However, a senior U.S. official said on Saturday an extension would be hard to consider without first seeing "significant progress on key issues."
"If (a comprehensive agreement) can't happen by July 20 both the (U.S.) administration and Congress are on the same page, which is that we obviously have to consider all of our options," said the official, who requested anonymity.
The French, British, and German foreign ministers joined Kerry and Ashton in Vienna.
Their Russian and Chinese counterparts will not attend due to a meeting of the so-called BRICS developing countries; Brazil, Russia, India, China, and South Africa. But Moscow and Beijing sent senior diplomats to Vienna for Sunday's talks.
Al Jazeera and wire services