Secretary of State John Kerry and Russian counterpart, Sergei Lavrov, discussed the idea of a ceasefire zone for Syria. (Reuters)

The top U.S. and Russian diplomats sparred publicly here Monday over whether Iran should be invited to attend Syria peace negotiations, while the special U.N. envoy hosting the talks again agreed with Russia that Iran should be included.

The Obama administration said as recently as Friday that Iran is not invited and will not be coming to the talks. But the U.N. envoy organizing the conference, Lakhdar Brahimi, said, “Iran is a very important country in the region, and they have to be present in a conference like this.”

Brahimi said the United States, Russia and the United Nations must reach consensus on which countries to invite. “Discussions are still continuing between the three, and we hope that before we are over, the right decision will be taken,” he said.

After a meeting here with Russian Foreign Minister Sergei Lavrov, Secretary of State John F. Kerry sought to lower expectations for early success in the negotiations between Syria’s government and opposition forces.

“We all understand that it will be difficult and will take some time,” Kerry said at a news conference with Lavrov and Brahimi. “But we must begin, and we must begin now.”

The Russian-backed government of Syrian President Bashar al-Assad reiterated Monday that it will attend the talks, although Syria has said repeatedly that it will not countenance negotiations on an end to Assad’s rule. Syria’s fractious political opposition, which has said that Assad and his associates cannot be part of a transition government, remains on the fence. Kerry, who met here Sunday with opposition leader Ahmad al-Jarba, reemphasized in a second meeting Monday “the importance of . . . sending a representative delegation to the peace negotiations in order to begin a process that would bring an end to the bloodshed and suffering in Syria,” a senior State Department official said.

Kerry, Lavrov and Brahimi agreed on the need for confidence-building measures before the scheduled Jan 22 talks in Switzerland between Syria’s warring sides. Among the measures being discussed with forces on the ground, the three diplomats said, are prisoner exchanges, localized cease-fires and an end to blockades by both sides that have prevented access for humanitarian aid to civilians.

Lavrov said the Syrian government has agreed to allow food and medical supplies to enter East Ghouta, a Damascus suburb that has been surrounded by forces loyal to Assad since early last year, as well as other areas. Lavrov called on the rebels to make similar concessions in Aleppo, which in recent weeks has been the site of fighting between rival rebel groups, even as government forces try to retake the northern city, Syria’s largest.

Kerry acknowledged the difficulty that U.S.-backed components of the rebel force have had dealing with extremist elements that are better armed and financed.

“The moderate opposition has said they will agree to a cease-fire,” Kerry said. “We have to begin a process to isolate the bad actors.” That, too, he said, is “going to take some time,” although there are reports this week that the moderates have driven extremist fighters out of Aleppo.

Kerry and Lavrov said they were in “full agreement” on the need to start the conference, which they first envisioned in May. But their animosity over the question of Iran’s participation was only thinly veiled.

Shiite Iran is one of Assad’s primary backers and has sent arms, advisers and, according to the United States, fighters to assist him in battling the rebels. Iran is also the primary sponsor of Hezbollah, a Lebanese Shiite militant group that has dispatched its own fighting force to Syria.

The Syrian opposition, in what has become a sectarian civil war, is largely Sunni Muslim.

The Obama administration has said that Iran is ineligible to attend the conference because it has not endorsed the goal of the negotiations, set 1 1 / 2 years ago in Geneva by the Friends of Syria group. The goal is to reach agreement, by mutual consent of the opposition and the Assad regime, on a transitional government to rule Syria until full democracy can be established.

The United States and the opposition have said the transitional government cannot include Assad.

The United States has been firm about keeping Iran out of the Geneva talks. “That’s right. They’re not coming,” a senior State Department official said during a briefing Friday.

Lavrov said Monday that he hopes U.N. Secretary General Ban Ki-moon will “make the right decision.”

“As usual, the U.N. is trying to put the responsibility on Russia and the U.S. But I do hope the secretary general is going to make the right decision,” Lavrov said. “After all, one cannot be influenced by ideological sentiments so much that it harms the interest of the cause.”

He said the United States and Iran had consulted in the past about the U.S. troop presence in Afghanistan and Iraq. “Back then, no one had any problems with ideology,” Lavrov said.

Kerry countered that it is “not a matter of ideology” but a “matter of common sense. We’ve asked [Iran] several times to simply state their support for the concept of mutual consent for the outcome of that conference.”

“I invite Iran today to join the community of nations . . . and be a constructive partner for peace,” he said. “That’s the invitation.”

Later in the news conference, all three diplomats returned to the issue.

“We still have a few days,” Brahimi said. “Definitely, we need an agreement, the three of us, on who is going to be invited or who is not going to be invited to the conference.”

The diplomats closed their session with the news media on a lighter note, with Kerry assuring a Russian reporter that his gift to Lavrov, of Idaho potatoes, had “no hidden meaning” beyond goodwill and good eating.

Lavrov suggested that perhaps the shape of the Idaho spuds “makes it possible to insert potato in the carrot-and-stick equation.”

On Tuesday, Kerry will travel to Rome to meet with Vatican Secretary of State Pietro Parolin.

Kerry,who is the first Catholic to hold the position of secretary of state since Edmund Muskie during the Carter administration, will discuss poverty, humanitarian issues and “Pope Francis’s vocal leadership on the Middle East peace process,” a State Department official said. Francis announced this month that he will visit the Holy Land in May.

The hastily arranged stop in Rome fills a gap in Kerry’s schedule, left when he postponed what would have been his 11th trip in 10 months to shepherd Israeli-Palestinian peace negotiations.

On Wednesday, Kerry will attend a conference in Kuwait of international humanitarian aid donors to Syria.

Sly reported from Beirut.