Highlights:

• Houthis targeted a Saudi airport, imposed a naval blockade
• Houthis say they responded to a Saudi attack on an airport
• UN special envoy seeks de-escalation, greater stability
• He says recent events are going in the opposite direction
• Pessimism grows over prospects for a negotiated resolution

Yemen is edging towards the resumption of a more than decade-old frozen conflict, Yemeni officials, Western diplomats and analysts say, as attacks on Saudi infrastructure and shipping by Iran-aligned Houthis test the patience of the kingdom and the Yemeni government forces it backs.

Houthi and Yemeni government forces have in recent days built up along a front line spanning the Houthi-controlled northwest of Yemen, running from the Red Sea coast to the Saudi border, Yemeni military officials and analysts said.

Mediation efforts led by the United Nations and Oman, and recently also involving Pakistan, have continued, but a Western diplomat said pessimism was growing about the prospects for a negotiated resolution.

Saudi Arabia has sought to avoid renewed conflict in Yemen since it struck a ceasefire with the Houthis in 2022, pausing a war that killed hundreds of thousands of people through fighting and famine and drew international outcry.

But the Houthi targeting of a Saudi airport last week and the announcement of a naval blockade of the kingdom — enforced with two attacks on Saudi tankers which the Houthis said they carried out — may be changing that calculus, the diplomats, analysts and Yemeni officials said.

The Houthis said the airport attack responded to a Saudi attack on an airport under their control and that the naval blockade was a response to a Saudi siege on Yemen, which Saudi Arabia has denied.

"Since 2022, these are the most serious preparations for a conflict I've seen," said Baraa Shiban, a Yemeni analyst and associate fellow at the Royal United Services Institute think tank in Britain.

The Saudi government media office, Yemen's information minister and a Houthi spokesperson did not respond to requests for comment.

US President Donald Trump said on Thursday he would hold Iran accountable for any Houthi attacks.

In a statement to Reuters, UN special envoy for Yemen Hans Grundberg said he had just returned from two trips to Gulf state Oman, with a trip to Saudi Arabia in between, "to discuss a way forward towards de-escalation and greater stability."

"Recent events have taken Yemen in the opposite direction, and that is a setback for Yemenis above all. My efforts continue, and the United Nations always stands ready to support the parties in reversing course and putting the interests of Yemenis first."

Iran and Houthis pile on pressure

Tensions are deepening in Yemen two weeks after the collapse of an interim truce meant to end the Iran war.

That war has all but shattered a 2023 detente between Saudi Arabia and Iran that had calmed regional tensions and allowed Saudi Crown Prince Mohammed bin Salman to focus on his Vision 2030 transformation plan, which includes huge infrastructure projects and the hosting of global events.

The Houthis have largely stayed on the sidelines of the Iran war but analysts, diplomats and Yemeni officials said the group and its Iranian backers now see an opening to pile pressure on both Saudi Arabia and the US.

The naval blockade on Saudi shipping could affect millions of barrels of oil a day pumped by pipeline to Saudi Arabia's Red Sea coast, a route used to skirt Iran's near-total blockade of the Strait of Hormuz.

For the Houthis, the escalation could also aim to extract financial and political concessions from Riyadh and Iran-style leverage over a global chokepoint by extending their reach over the strait known as Bab al-Mandeb, Shiban said.

A senior Houthi official said Bab al-Mandeb, which controls access from the Red Sea to the Indian Ocean, remained open to non-Saudi shipping.

Saudi frustration builds with Houthis

The recent tensions fly in the face of years of Saudi engagement with the Houthis that awarded them concessions including use of Hodeidah port and the 2024 reversal of plans to block all international banking access in areas they control.

"There is increasingly a feeling that peace with the Houthis is unattainable, and that a delayed war might prove costlier still, especially with Saudi Arabia set to host Expo 2030 and the (soccer) World Cup in 2034," said Farea al-Muslimi, a research fellow at the Chatham House think tank's Middle East and North Africa programme.

"It's risky, but it could help clear the way," he said, adding that US support would be critical.

The political landscape has shifted in Saudi Arabia's favour since it first intervened in 2015 at the head of an Arab coalition, when the killing of thousands of civilians in bombing raids drew US and European pressure on Riyadh to end the war.

Houthi attacks on Israel and international shipping since the 7 October 2023 attack on Israel by the Palestinian militant group Hamas, along with the detention of dozens of UN staff, have hardened international opinion against the Houthi movement, analysts and Yemeni government officials said — a shift they say works in Saudi Arabia's favour.

Should Riyadh intervene again, "there will be much, much less scrutiny," said Shiban.