Iran's supreme leader Ayatollah Ali Khamenei on Thursday accused Saudi Arabia of "genocide" in Yemen and urged the Arab monarchy to stop its "disastrous crimes as soon as possible," as escalating rhetoric compounded regional tensions.
The unusually harsh comments directed at Tehran's main regional rival come after a fortnight of Saudi-led air strikes against Shia Houthi rebels in Yemen, who have been branded by Riyadh as Iranian-backed terrorists.
"Killing children and destroying the infrastructure and national wealth of a country is a big crime . . . Saudis will definitely lose," Ayatollah Khamenei said in a speech. "This move in the region is not acceptable and I warn them to stop their criminal act in Yemen."
The escalating war of words comes amid increasing military posturing around the war in the south-western corner of the Arabian Peninsula.
Iran on Wednesday moved two naval vessels into the seas off Yemen, where much of the world's crude oil passes from the Indian Ocean into the Red Sea via the potential chokepoint of the Bab el-Mandab straits.
The US, which last week struck a provisional agreement with Tehran to ease sanctions in return for limits on Iran's nuclear programme, is supporting the Saudi-led coalition of 10 Sunni Muslim states, saying it will speed the provision of arms and intelligence.
Ayatollah Khamenei lashed out at the US support for Riyadh and said it was normal for the US to back the "oppressor and not the oppressed".
The supreme leader also denied that Iran was intervening in Yemen, describing such claims as a "foolish excuse" by "those whose criminal jets have made Yemen's sky insecure . . . to interfere in Yemen".
But he added that Yemeni fighters from all sects, both Shia and Sunni, would continue to resist aggression, hinting that Iran would continue to support the Houthi rebels.
The Houthis, who are said to be allied to Iran, have been calling for greater rights for their minority Zaydi Shia community based in northern Yemen. Over the past few years, they have expanded through northern Yemen westwards to the Red Sea coast and south into the capital. The rapid rise of the Houthis has alarmed neighbouring Saudi Arabia, which is locked in a proxy cold war with Iran for regional dominance. Riyadh sees the rebel gains as extension of the Islamic republic's reach in the region. Tehran already has proxies in Syria, Iraq and Lebanon.
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Iran's president, Hassan Rouhani, also warned Saudi Arabia and its Arab allies to remember the fate of ousted Iraqi president Saddam Hussein and realise that "aggression is not the answer."
"You will soon realise you made a mistake in Yemen as you did in Lebanon, Syria and Iraq," he said in a live televised speech. "A great nation like Yemen will not submit to bombardments; do not kill innocent children; let us all think about end to war and establishing a ceasefire."
Despite the growing rhetoric, Gulf leaders are considering whether to deploy ground troops to complement the aerial campaign.
Abdullah bin Zayed, the United Arab Emirates' foreign minister, on Wednesday said the coalition would not place limits on its military operation to restore legitimacy to government in Yemen, and Saudi Arabia has asked ally and coalition member Pakistan for troops and equipment to aid its campaign in Yemen.
In another sign of rising tensions between Riyadh and Tehran, which have been facing off against one another in increasingly sectarian proxy conflicts across the Middle East, Saudi authorities on Thursday denied entry to an Iranian plane carrying pilgrims to the kingdom.
Iranian parliamentarians and clerics have in turn called for the government to stop state-organised pilgrimages to Saudi Arabia.
The Houthis, with former president Ali Abdullah Saleh, have expanded their control from the capital into the south, unseating the president, Abd-Rabbu Hadi, who has fled to Riyadh.
Houthi forces have over recent days been engaged in fierce battles with troops loyal to Mr Hadiin the southern port city of Aden, where aid agencies say a humanitarian crisis is unfolding.
Iran's top leader even went as far as comparing Saudi Arabia's war on Yemen with Israeli strikes on Gaza.
"The reason for this prediction [of failure for the Saudis] is obvious because Zionists' military capabilities are far greater than the Saudis' and Gaza was a small area," he said. "But . . . Yemen is a big country with tens of millions of population . . . Saudis' noses will surely be rubbed into the soil."
He blamed "a couple of immature young [politicians in Saudi Arabia who] have taken over the country's affairs and are replacing dignity [in foreign policy] with primitivism".
This would appear to be a reference to 30-year-old Mohammed bin Salman, the son of King Salman, who was appointed as defence minister when the king ascended to the throne in January. The young prince's leading role in coordinating the air campaign has been highlighted by Saudi media.