Iran's foreign minister sought Friday to dismiss concerns that his country's preoccupation with the crisis in Yemen could pre-empt attempts to find common ground at nuclear talks with six world powers, saying the negotiations remained focused on sealing a deal.

Yemen is "the hot issue of the day" and has come up at the talks but "it doesn't mean that we negotiated about it," Mohammad Javad Zarif told reporters.

Saudi-led air strikes on Shiite rebels in Yemen are straining relations between the Sunni Gulf kingdom and predominantly Shiite Iran. Zarif said they "have to stop and everybody has to encourage dialogue and national reconciliation."

Despite Iran's concerns over Yemen, however, "our negotiations are confined to the nuclear" issue, he said.

Zarif spoke shortly after his first meeting of the day with U.S. Secretary of State John Kerry. The sides are hoping to narrow gaps in time to reach a preliminary deal by the end of the month. That would allow them to try and negotiate a comprehensive agreement by late June to put long-term curbs on Tehran's nuclear activities in exchange for sanctions relief.

Iranian officials have been upbeat recently about the chances of making enough progress by Tuesday to permit them to proceed into the summer. But Zarif was less bullish Friday, saying only that he hoped the sides would come to a common understanding by next week.

"The talks are very difficult and very complicated," he told Iranian TV.

In Washington, British Foreign Secretary Philip Hammond said the parties were "better than half way" to a deal.

On some issues "we're close enough that we can be sure we will reach agreement on them," Hammond said. But on others, he added, "we are significantly apart and it's going to require a significant move by the Iranians to reach our red line positions."

The Obama administration has made an accord that lessens fear about Iran's nuclear weapons potential a top foreign policy objective. Iran denies any interest in such arms but has been drawn to the negotiating table in part by the pressure of crippling economic sanctions.

Reflecting Tehran's interest in reaching a deal, Iranian President Hassan Rouhani sent a letter to President Barack Obama and the leaders of the other countries at the talks -- Russia, China, Britain, France and Germany. His office said Friday the letters contained proposals on how to reach a deal, without elaborating. Rouhani also spoke to the leaders of Russia, France and Britain by phone.

The fate of a fortified underground bunker previously used for uranium enrichment appeared close to resolution. Officials have told The Associated Press that the U.S. may allow Iran to run hundreds of centrifuges at the formerly secret facility in exchange for limits on centrifuge work and research and development at other sites.

The trade-off would allow Iran to run several hundred of the devices at its Fordo facility, although the Iranians would not be allowed to do work that could lead to an atomic bomb and the site would be subject to international inspections.

In return, Iran would be required to scale back the number of centrifuges it runs at its Natanz facility and accept other restrictions on nuclear-related work.

Instead of uranium, which can be enriched to be the fissile core of a nuclear weapon, any centrifuges permitted at Fordo would be fed elements such as zinc, xenon or germanium for separating out isotopes used in medicine, industry or science, the officials said.

Even if repurposed to enrich uranium, the number of centrifuges would not be enough to produce the amount needed to produce a weapon within a year -- the minimum time-frame that Washington and its negotiating partners demand.