For diplomats from countries without diplomatic relations, Secretary of State John Kerry and Iranian Foreign Minister Mohammad Javad Zarif sure are doing a lot of diplomacy.

As Iran races to satisfy the terms of last summer's nuclear deal and the U.S. prepares to suspend sanctions on Tehran as early as Friday, Kerry is talking to Zarif more than any other foreign leader. Those talks included several emergency calls Tuesday to secure the release of 10 U.S. sailors after Iran detained them in the Persian Gulf.

Since the beginning of the year, Kerry and Zarif have spoken by phone at least 11 times, according to the State Department. They've focused on nuclear matters, Iran's worsening rivalry with Saudi Arabia and peace efforts in Syria.

By contrast, America's top diplomat has talked to Saudi Foreign Minister Adel al-Jubeir only twice. He has consulted once each with Saudi Deputy Crown Prince Mohammed bin Salman, Jordan's King Abdullah and the foreign ministers of Britain, Egypt, France, Germany, Russia and the European Union.

Kerry departed Wednesday evening to meet al-Jubeir in London. He may extend the trip to see Zarif, too, elsewhere in Europe.

Not everyone is happy with the new friendship between the once hostile foes. But the White House, Pentagon, Kerry and Zarif are all crediting the relationship forged over two-and-a-half years of nuclear negotiations with quickly resolving the detention of the sailors, which could have been a new crisis just as President Barack Obama delivered his final State of the Union speech to Congress.

"We can all imagine how a similar situation might have played out three or four years ago, and the fact that today this kind of issue can be resolved peacefully and efficiently is a testament to the critical role diplomacy plays in keeping our country safe, secure and strong," Kerry said Wednesday.

Denis McDonough, the White House chief of staff, said the open lines between Kerry and Zarif are "extraordinarily important." Defense Secretary Ash Carter thanked Kerry for his efforts. And in Tehran, Zarif tweeted that he was "happy to see dialogue and respect, not threats and impetuousness, swiftly resolved the sailors episode. Let's learn from this latest example."

For the Obama administration, the budding Kerry-Zarif relationship offers opportunities and pitfalls. As negotiations with Tehran accelerated in 2013, U.S. officials insisted the diplomacy concerned only ending the threat of a nuclear-armed Iran and shouldn't signal a strategic realignment that would forsake longstanding U.S. allies like Israel or the Gulf state Sunni monarchies.

As the deal came together last July, Obama and his top aides vowed to "double down" on Iran's activities like its support for Syrian leader Bashar Assad's government and anti-Israel and anti-U.S. groups like Hamas and Hezbollah.

The record, however, has been mixed.

The U.S. has modestly expanded sanctions on Hezbollah, but hasn't followed through on pledges to impose penalties after a recent ballistic missile test by Iran that violated a U.N. Security Council ban. Washington also offered no response to an Iranian navy rocket fired near a U.S. warship in the Strait of Hormuz.

At the same time, the U.S. has expanded outreach efforts to Iran. The Obama administration included the Iranians for the first time in international mediation efforts to secure a cease-fire between Assad's government and rebel groups.

Echoing Saudi and Israeli concerns, Republican lawmakers say they're unsettled by the warming ties. They say the nuclear deal that they opposed is preventing Obama from dealing assertively with Iran, because he is more concerned with protecting his signature foreign policy achievement.

Conscious of these perceptions, the administration moved warily into the Saudi-Iran spat that erupted over New Year's weekend. The U.S. did not want to play mediator between a longtime ally in Riyadh and a government that, at least officially, remains hostile to the United States some three-and-a-half decades after the 1979 Islamic Revolution and U.S. Embassy hostage crisis.

But the U.S. has a lot at stake in calming tensions. Saudi Arabia and Iran already are backing opposing sides in Syria and refusing to cooperate against the Islamic State. They're trading accusations over the war in Yemen, where a Saudi-led intervention hasn't uprooted the Iran-backed Houthi rebels.

If the Saudi-Iranian crisis escalates, diplomats fear the newest stab at Syria peace talks could unravel before the scheduled start later this month. And unlike previous efforts, Washington now believes uniting Assad and rebels in a unity government is critical for defeating the Islamic State.

The U.S. is banking on its new dynamic with Iran, and particularly Kerry's relationship with Zarif.

The secretary of state is trying to "de-escalate the tensions, restore some sense of calm, encourage dialogue and engagement between these countries bilaterally, but also to make the point that there are other pressing issues in the region," State Department spokesman John Kirby said. He spoke last week after the Saudis executed a leading Shiite cleric, an Iranian mob stormed the Saudi Embassy in Tehran, and the Saudis and some other Sunni countries severed diplomatic ties with Iran.

For Zarif, the regular exchanges with Kerry are something of a liability at home. Iranian hardliners have criticized him for overstepping his limited mandate to speak with Kerry about nuclear matters.

While he and Kerry exchanged a "few words" about the Saudi-Iranian dispute, Zarif said last week, "it does not mean that we have launched negotiations with the U.S."

The conversations are likely to broaden, even though the U.S. still considers Iran the leading state sponsor of terrorism and Iranian officials routinely denounce a nation they sometimes call "the Great Satan."

Tehran is set to satisfy its obligations from July's nuclear accord in the coming days, diplomats reported Wednesday, obliging Washington to open up lucrative oil, trade and financial opportunities for the Iranians. As they make good on their promises, both Kerry and Zarif have raised the prospect of even more engagement to come.