The scheduled talks come a day after Kerry and the foreign ministers of Britain, France and Germany failed to reach a breakthrough on uranium enrichment and other issues standing in the way of a deal that would curb Iran's nuclear program in exchange for the end of nuclear-related sanctions on Tehran.
The top officials took turns meeting with Iranian Foreign Minister Mohammed Javad Zarif, and each gave an assessment describing significant gaps between the two sides. Russia and China sent lower-level officials to Austria's capital for this week's gathering.
Six months ago, the six world powers and Tehran gave themselves until July 20 to conclude what is supposed to be a multi-decade agreement that sets clear limits on Iranian activity and locks in place an international monitoring regime designed to ensure that the Islamic republic cannot develop nuclear weapons.
But the interim agreement also provides the option of an additional six-month window for hammering out a full accord, though officials have suggested a shorter extension may be agreed upon.
Kerry's second day of talks will continue his efforts to gauge "Iran's willingness to make the critical choices it needs to make," according to a senior State Department official.
The official didn't say how long Kerry's discussions with Zarif would run, but indicated a "potentially lengthy conversation" lay ahead. The official wasn't authorized to be quoted by name and demanded anonymity.
On Sunday, British Foreign Minister William Hague said no breakthrough had been reached. But Kerry said he was not giving up.
"We're working, we're working, we just got here," said Kerry, chiding reporters asking about progress as the day's meetings wound down.
Zarif said no problems had been resolved "but I think we have made some important headway."
Iran says it needs to expand enrichment to make reactor fuel and insists it does not want atomic arms. But the US and others fear Tehran could steer the activity toward manufacturing the core of nuclear missiles. Washington is leading the charge for deep Iranian enrichment cuts.
Germany's Frank-Walter Steinmeier and France's Laurent Fabius left Sunday, a few hours after they arrived. But Hague joined Kerry in staying on for another day.
The show of Western unity notwithstanding, Kerry's presence was most important. With the most significant disputes between Washington and Tehran, his visit gave him a chance to discuss them directly with Zarif.
Both face difficult internal pressures.
Iranian hardliners oppose almost any concession by moderate President Hassan Rouhani's government. In the US, Republicans and Democrats have threatened to scuttle any emerging agreement because it would allow Iran to maintain some enrichment capacity.
Outside the negotiations, regional rivals of Iran, including Israel and Saudi Arabia, are extremely skeptical of any arrangement they feel would allow the Islamic republic to escape international pressure while moving closer to the nuclear club.
Iranian Deputy Foreign Minister Abbas Araghchi suggested any extension would be relatively short, saying "there is not much willingness" by either side to go a full six months. He, too, spoke Sunday of "huge and deep differences."