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 Shiite Iran, which 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 Foreign Minister Adel al-Jubeir also warned Tehran.
"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.
Iran's Foreign Minister Mohammad Javad Zarif issued dismissive tweets over the kingdom of Saudi Arabia in response.
"KSA bombs Yemen to smithereens, killing 1000s of innocents including babies, spreads cholera and famine, but of course blames Iran," he wrote.
"KSA is engaged in wars of aggression, regional bullying, destabilising behaviour & risky provocations. It blames Iran for the consequences."
Saudi forces on Saturday intercepted and destroyed the ballistic missile near Riyadh's international airport after it was reportedly fired by Shiite Huthi 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 President Abedrabbo Mansour Hadi's internationally recognised government since 2015.
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 Huthis 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.
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 because of the "grip" of Iran and its ally Hezbollah on the country.
Hezbollah chief Hassan Nasrallah hit back 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, on Monday met Saudi King Salman and "reviewed the situation in Lebanon", the state-run Saudi Press Agency reported.
Saudi Arabia accused Lebanon on Monday of declaring war against it because of aggression by the Iran-backed Lebanese Shi‘ite group Hezbollah, a dramatic escalation of a crisis threatening to destabilize the tiny Arab country.
Lebanon has been thrust to the center of regional rivalry between Saudi Arabia and Iran since the Saudi-allied Lebanese politician Saad al-Hariri quit as prime minister on Saturday, blaming Iran and Hezbollah in his resignation speech.
Saudi Gulf affairs minister Thamer al-Sabhan said the Lebanese government would “be dealt with as a government declaring war on Saudi Arabia” because of what he described as aggression by Hezbollah.
Faulting the Hariri-led administration for failing to take action against Hezbollah during a year in office, Sabhan said “there are those who will stop (Hezbollah) and make it return to the caves of South Lebanon”, the heartland of the Shi‘ite community.
In an interview with Al-Arabiya TV, he added: “Lebanese must all know these risks and work to fix matters before they reach the point of no return.”
He did not spell out what action Saudi Arabia might take against Lebanon, a country with a weak and heavily indebted state that is still rebuilding from its 1975-90 civil war and where one-in-four people is a Syrian refugee.
Hezbollah is both a military and a political organization that is represented in the Lebanese parliament and in the Hariri-led coalition government formed last year.
Its powerful guerrilla army is widely seen as stronger than the Lebanese army, and has played a major role in the war in neighboring Syria, another theater of Saudi-Iranian rivalry where Hezbollah has fought in support of the government.
Lebanese authorities said on Monday the country’s financial institutions could cope with Hariri’s resignation and the stability of the Lebanese pound was not at risk.
But the cash price of Lebanon’s U.S. dollar-denominated bonds fell, with longer-dated maturities suffering hefty losses as investors took a dim view of the medium- to longer-term outlook for Lebanon.