Saudi Arabia and Iran traded fierce accusations over Yemen on Monday, with Riyadh saying a rebel missile attack “may amount to an act of war” and Tehran accusing its rival of war crimes.

Tensions have been rising between Sunni-ruled Saudi Arabia and predominantly Shia Iran, who are opposed in disputes and conflicts across the Middle East, from Yemen and Syria to Qatar and Lebanon.

On Monday, a Saudi-led military coalition battling Tehran-backed rebels in Yemen said it reserved the “right to respond” to the missile attack on Riyadh at the weekend, calling it a “blatant military aggression by the Iranian regime which may amount to an act of war.”

Saudi forces on Saturday intercepted and destroyed the ballistic missile near Riyadh's international airport after it was reportedly fired by the Houthi rebels from Yemen.

It was the first attempted missile strike by the rebels to reach Riyadh and threaten air traffic, underscoring the growing threat posed by the conflict on Saudi Arabia's southern border.

The coalition on Monday sealed off air, sea and land borders in Yemen, where it has been battling rebels in support of the President Abedrabbo Mansour Hadi's internationally recognised government since 2015.

Saudi warns Iran it will not tolerate 'any infringement' of its security

Saudi Foreign Minister Adel al-Jubeir warned Tehran the kingdom would not tolerate “any infringement” on its national security.

“Iranian interventions in the region are detrimental to the security of neighbouring countries and affect international peace and security. We will not allow any infringement on our national security,” Jubeir tweeted.

“The kingdom reserves the right to respond in a timely manner to the hostile actions of the Iranian regime,” he added, echoing earlier warnings by a Saudi-led military coalition in Yemen.

Iran rejects Saudi accusations

Iran dismissed the Saudi accusations, saying rebels fired the missile in retaliation for Saudi “war crimes”.

An Iranian foreign ministry statement quoted spokesman Bahram Ghassemi as saying the accusations by the coalition were “unjust, irresponsible, destructive and provocative”.

Ghassemi said the missile was fired by the Houthis in response “to war crimes and several years of aggression by the Saudis.”

The missile attack, he said, was “an independent action in response to this aggression,” and Iran had nothing to do with it.

He also called on Riyadh to halt attacks on “defenceless and innocent people as soon as possible and to pave the way for inter-Yemeni dialogue to bring peace to the country.”

Rewards for rebel leaders

Critics have accused the coalition of not doing enough to prevent civilian deaths in its air war in Yemen, where more than 8,650 people have been killed since the start of the intervention.

Repeated attempts to bring about a negotiated settlement to the conflict have failed, including a series of UN-backed peace talks, and positions appear to have hardened in recent months.

Saudi Arabia has blamed the Huthis for the failed efforts, and on Monday offered rewards totalling $440 million for information on 40 senior officials among the rebels.

Topping the list, with a $30m reward for tips leading to his capture, was the group's leader Abdulmalik al-Houthi.

The Houthis have allied with Yemen's ex-president Ali Abdullah Saleh in the conflict, forcing Hadi's government to operate from the southern city of Aden.

Analysts said it was unclear how far Saudi Arabia would be willing to go in the escalating confrontation.

The kingdom is in the midst of an unprecedented purge of its upper ranks, with dozens of senior figures arrested at the weekend, as 32-year-old Crown Prince Mohammed bin Salman consolidates his hold on power.

Prince Mohammed, who is also defence minister, is seen as a key supporter of the intervention in Yemen.

Analyst Randa Slim of the Middle East Institute said it was unclear whether the Saudi leadership had “thought through an escalation of the scale they're hinting at.”

Compounding concerns of an escalation, she said, is that US President Donald Trump's administration has also taken a hard line against Iran “and may not send a deterrent message to Saudi.”

The Saudi-Iran rivalry also played out this weekend in the resignation of Lebanon's prime minister Saad Hariri, a protege of Riyadh who said he was stepping down due to the “grip” of Iran and its ally Hezbollah on the country.

Hezbollah chief Hassan Nasrallah hit back in a somber speech on Sunday, saying Hariri's resignation had been “imposed” by Saudi Arabia.

Iran rejected Hariri's words as “baseless” and said his resignation was “designed to create tensions in Lebanon and in the region”.

Hariri, who announced his resignation in Riyadh, met on Monday with Saudi King Salman, where they “reviewed the situation in Lebanon,” according to the state-run Saudi Press Agency.