SANAA/CAIRO: The Iran-backed Houthi rebels in Yemen threatened Tuesday to attack ports and airports in Saudi Arabia and the United Arab Emirates, raising the stakes in a flare-up between Riyadh and Tehran.

The threat came hours after Saudi Arabia’s powerful crown prince accused Iran of “direct military aggression” through its support for the rebels – a claim Tehran rejected as “contrary to reality.”

“The involvement of Iran in supplying missiles to the Houthis is a direct military aggression by the Iranian regime,” the official Saudi Press Agency quoted Crown Prince Mohammad bin Salman as saying.

This “could be considered an act of war,” he said.

Iranian Foreign Minister Mohammad Javad Zarif retorted that “the allegations by Saudi officials were contrary to reality,” a Foreign Ministry spokesman said.

The soaring tensions between the key oil producers saw crude trading at close to two-year highs Tuesday and spooked Gulf markets.

The rebels already showed Saturday that despite a more than two-year bombing campaign by a Saudi-led coalition, they retain missiles capable of striking targets deep inside the kingdom.

A rebel missile was intercepted and destroyed near Riyadh international airport – the first to reach the Saudi capital – with smoldering debris inside the perimeter, underscoring the growing fallout for Saudi Arabia from its involvement in neighboring Yemen.

“All airports, ports, border crossings and areas of any importance to Saudi Arabia and the UAE will be a direct target of our weapons, which is a legitimate right,” the rebels’ political office said in statement.

“We will not stand idly by – we will seek more radical means to prevent both the tightening of the blockade and all acts aimed at starving and humiliating the people of Yemen.”

Saudi Arabia and the UAE are the two major powers in a coalition that has been fighting the Yemeni rebels since 2015 in support of the internationally recognized government.

Since Saturday’s missile attack, the coalition has tightened its blockade of rebel-held areas of Yemen, blocking even U.N.-supervised relief supplies despite urgent appeals from the world body.

The coalition said its action was aimed at filling the gaps in inspection procedures that enable “smuggling of missiles and military equipment” to the rebels.

But the blocking of all relief supplies further threatens some 7 million people already on the brink of famine and the U.N. urged the coalition to lift it as soon as possible.

“If these channels, these lifelines, are not kept open, it is catastrophic for people who are already in what we have already called the world’s worst humanitarian crisis,” said Jens Laerke, a spokesman for the U.N. humanitarian office (OCHA) in Geneva.

OCHA said the coalition had also asked it to clear ships from the rebel-held Red Sea port of Hudaida, the backbone of its humanitarian operations in Yemen. It said it was in talks with the coalition to restore access as soon as possible.

Laerke said that in the immediate aftermath of the blockade, fuel prices in rebel-held areas had jumped by up to 60 percent and cooking gas prices had doubled.

The International Committee of the Red Cross also urged the reopening of ports for medical supplies. The relief agency said a shipment of chlorine tablets used to prevent cholera, which has ravaged Yemen in recent months, did not get a clearance at Yemen’s northern border. More supplies are due next week, including 50,000 vials of insulin, the ICRC said.

“Insulin cannot wait at a shuttered border since it must be kept refrigerated. Without a quick solution to the closure, the humanitarian consequences will be dire,” the ICRC’s Regional Director Robert Mardini said.

In announcing the closures earlier this week, Saudi Arabia said it would take into consideration continuing aid efforts.

Later Tuesday, Yemeni officials said suspected coalition air raids killed at least 23 people in the rebel-controlled northern province of Al-Hajjah. They said the strikes targeted homes of local sheikhs where the head of the Houthis’ Supreme Political Council, Saleh al-Sammad, was visiting.Houthi spokesman Mohammad Abdel-Salam denied Saudi media reports that Sammad was killed.