CAIRO — Tensions between Iran and Saudi Arabia deepened on Thursday as Iranian leaders lashed out with rare vehemence against the continuing Saudi air campaign in Yemen, even hurling personal insults at the young Saudi prince who is leading the fight.
Ayatollah Ali Khamenei, the supreme leader of Iran, on Thursday denounced the Saudi airstrikes in Yemen as “a crime” and “a genocide,” while all but taunting Saudi Arabia that its war in Yemen was doomed to fail.
A regional coalition led by Saudi Arabia extended its bombing campaign for a 16th night in its effort to stop the Houthi movement and its allies from dominating Yemen. The Houthis nonetheless continued their advance, and aid groups warned of a compounding humanitarian catastrophe, particularly in the port city of Aden.
Secretary of State John Kerry sharply warned Iran over its backing for the other side of the conflict in Yemen, in the first explicit American accusation that Tehran has been providing military aid to the Houthis.
Washington was “not going to stand by while the region is destabilized,” Mr. Kerry said in an interview with “PBS NewsHour” on Wednesday night.
“There are obviously supplies that have been coming from Iran,” he added. “There are a number of flights every single week that have been flying in. We trace those flights, and we know this. We are well aware of the support that Iran has been giving to Yemen.”
The United States has recently increased the provision of logistical support, intelligence and weapons to the Saudi campaign, just days after the announcement of a framework for a nuclear deal with Iran. Mr. Kerry said he was seeking to reassure allies, including Saudi Arabia, that the United States could “do two things at the same time.” The United States could help push back against Iranian attempts to project its influence around the region, he argued, while at the same time negotiating a deal that would reward Tehran for providing guarantees that it was not building nuclear weapons.
Saudi Arabia has said it is bombing the Houthis because it sees them as an instrument of Iranian power, but Western diplomats and independent experts have said that while Iran has supported the Houthis financially, it has no direct control over the group.
The back-and-forth insults and threats Thursday raised fears that the Yemeni conflict could further inflame the rivalry between the two regional heavyweights, Iran and Saudi Arabia, already fighting each other through sectarian proxies in Syria and in less overt struggles across Iraq, Lebanon and Bahrain.
Some analysts suggested that the battle for Yemen may further complicate the delicate politics of a deal with the Western powers to restrain Iran’s nuclear program. But others argued that the deal had instead emboldened Iran to flex its muscles against Saudi Arabia.
“Deep down, the Iranians know that they are winning,” said Michael Stephens, the head of the Royal United Services Institute in Doha, Qatar.
Mr. Kerry appeared forced into a difficult balance, reassuring Saudi Arabia and other allies of American support against Iran without antagonizing Iranian hard-liners severely enough to fuel opposition to the nuclear deal.
The Ayatollah’s diatribe only added to the challenge.
In rare direct criticism of Iran’s rival by name, Ayatollah Khamenei said that Saudi Arabia was departing from a history of acting with dignity in foreign affairs, saying that “inexperienced youths have taken over the affairs of the state and are replacing dignity with barbarity.”
His comments were a jab at the Saudi defense minister, Prince Mohamed bin Salman, who Saudi diplomats say is about 30. He is also a son of King Salman, who ascended to the throne this year and promptly named him to the powerful dual roles of defense minister and chief of the royal court.
The Saudi news media has been cheering Prince Mohamed as the architect and overseer of the Yemeni campaign despite a light résumé, raising eyebrows among rivals in the royal family and setting him up for embarrassment if it is deemed to have failed.
In a statement on Thursday, Ayatollah Khamenei warned the Saudis “they must cease their crimes in Yemen,” and that failure there was all but inevitable.
“They will be harmed and incur losses in this issue in which they will under no circumstances triumph. The Saudis’ face will be rubbed in the ground in Yemen,” he said, comparing the Saudi actions to Israel’s campaigns against Palestinian militants in Gaza — who have also received support from Iran.
In another statement, released over Twitter along with some of his other remarks, the ayatollah further teased Saudi Arabia about its pledge to catch up to Iran in the development of a nuclear weapon.
“An underdeveloped country said that ‘If Iran has enrichment, we want it too,’ ” he said. “Well, do it if you can. Nuclear technology is our domestic capability.”
Mr. Stephens, of the Royal United Services Institute, said the extraordinary barbed comments appeared designed to provoke and inflame.
“It is almost like the Iranians are baiting the Saudis, trying to find every pressure point, to make the Saudis feel emasculated, and then stand back and watch as the Saudis get in deeper and deeper,” he said.
But at the same time, he argued, the Iranians were sending a message of support to their Yemeni allies, the Houthis.
“They have decided to show that they are willing to stand behind the Houthis,” Mr. Stephens said. “The Iranians are making some sort of a guarantee to their allies that, whatever happens, they are going to make sure that the Houthis are at the table for the talks.”
Saudi Arabia, which is leading a regional coalition against the Houthis, issued its own warning to Iran, saying that two Iranian warships that recently arrived in the waters off Yemen should steer clear of Yemeni waters. “If the ships seek to aid the Houthis, the coalition has the right to choose the proper answer,” said a spokesman, Brig. Gen. Ahmad Asseri.
In another provocation, Saudi Arabia denied permission for an Iranian plane filled with pilgrims to land in Mecca.
The Houthis, a group based in northern Yemen that follows a strain of Shiite Islam, have fought a half dozen conflicts with the Yemeni government since 2004, including a fight in 2009 that drew Saudi Arabia into border skirmishes. Only since that time, scholars say, has the movement received support from Iran, the region’s main Shiite Muslim power as well as Saudi Arabia’s rival.
Since the fall, however, the Houthis have teamed with major units of the Yemeni security forces still loyal to the former strongman, Ali Abdullah Saleh. Those units have helped the Houthi-allied forces capture the capital, Sana, and much of Yemen.
Their advance appears to have continued despite the two-week-old Saudi bombing campaign. On Thursday, Al Jazeera television network — owned by Qatar, part of the Saudi coalition — said the Houthi-allied forces had also occupied Ataq, capital of Shabwa Province, taking control of government offices.
At the United Nations, Secretary-General Ban Ki-moon, whose efforts to restart a political dialogue in Yemen have collapsed, implored the Houthis to halt their expansion. But he also has been critical of the Saudi-led military campaign.
“The coalition air raids — and the continuing attempts by the Houthis and their allied armed groups to expand their power — have turned an internal political crisis into a violent conflict that risks deep and long-lasting regional repercussions,” Mr. Ban said at United Nations headquarters.
“The last thing the region and our world need is more of the chaos and crimes we have seen in Libya and Syria,” he said.
Thomas Erdbrink contributed reporting from Tehran, Rick Gladstone from New York, and Kareem Fahim from Cairo.