BEIRUT (NYTIMES) - When a band of scrappy rebels known as the Houthis stormed out of the mountains of northern Yemen in 2014 and took over the capital Sanaa, their friends and foes alike dismissed them as unsophisticated tribal fighters running around in sandals and armed with cheap guns.

But during the civil war that has shattered Yemen in the years since, the group has gone through a remarkable transformation. It now rules a repressive proto-state in northern Yemen and wields a vast arsenal that includes an array of cruise and ballistic missiles and kamikaze boats.

The Houthis also assemble their own long-range drones, which have extended their reach across the Arabian Peninsula and amplified threats to Persian Gulf powerhouses Saudi Arabia and the United Arab Emirates, both partners of the United States and leaders of the coalition that has waged war against the Houthis since 2015.

The swift expansion of the Houthis' abilities is largely thanks to covert military aid from Iran, according to American and Middle Eastern officials and analysts.

Seeking new ways to menace Saudi Arabia, its regional nemesis, Iran has integrated the Houthis into its network of militias and built up the Houthis' ability to subvert their wealthy neighbours' defences with relatively cheap weapons. And many of those weapons are now built in Yemen, the Arab world's poorest country.

"What we are seeing in Yemen is technology being the great equaliser," said Mr Abdulghani Al-Iryani, a senior researcher at the Sanaa Centre for Strategic Studies.

Summarising the Houthi mindset, he said, "Your F-15 that costs millions of dollars means nothing because I have my drone that cost a few thousand dollars that will do just as much damage."

The Houthis' advancing military technology has added new urgency to Saudi efforts to end the war seven years after intervening. But those advances may also have made the Houthis less interested in ending it, even though they agreed to a two-month cease-fire that began at the start of this month, aimed at kick-starting peace talks.

Saudi Arabia and the UAE have also thrown their support behind a new presidential council formed this month to run the Yemeni government and lead negotiations with the Houthis.

Still, in the first three months of this year, the Houthis demonstrated the threat they posed to Persian Gulf countries.

Attacks launched from Yemen killed three workers at a fuel depot in Abu Dhabi, the capital of the UAE; put American troops in the UAE on alert while US and Emirati forces deployed costly defence systems to shoot down incoming missiles; and ignited an oil facility in western Saudi Arabia, filling the sky over a Formula One car race with thick black smoke.

The war has deepened the Houthis' relationship with their powerful backer, Iran, allowing them to develop a vast war economy to fund their operations.

It has also made them the uncontested authority over a large section of northern Yemen, where more than two-thirds of the country's population lives - gains they are unlikely to give up voluntarily, analysts said.

"If the war stops, the Houthis will have to govern, and they don't want to govern - to provide services and share power," said Ms Nadwa Al-Dawsari, a Yemen analyst at the Middle East Institute. "The Houthis thrive in war, not peace."

The Houthis, officially known as Ansar Allah, or the Partisans of God, honed their guerrilla abilities during a series of brutal battles with the Yemeni state and Saudi Arabia in the 2000s. Those conflicts bolstered their sense of themselves as underdogs defending Yemen from more powerful aggressors.

Their slogan - "Death to America. Death to Israel. Curse on the Jews. Victory for Islam." - is splashed on posters across their territory and screamed at protests.

In 2014, the Houthis seized Sanaa, proclaiming that they sought to stamp out corruption. A Saudi-led military coalition intervened against them in early 2015, launching a bombing campaign aimed at restoring the internationally recognised government that the Houthis had driven into exile.

As the war settled into a grinding stalemate and festering humanitarian crisis, Iran quietly ramped up its support for the Houthi war machine.

Houthi technicians flew to Iran for training, and experts from Iran's Revolutionary Guard and Lebanon's Hezbollah travelled to Yemen to organise the group's fighters and media teams and, later, to teach Houthi technicians how to build weapons, according to members of the Iranian axis in the region and analysts tracking the conflict.

Early in the war, the Houthis mostly hit back at Saudi Arabia by striking targets along the Saudi border with northern Yemen. But the reach and sophistication of their weapons have increased rapidly, enabling them to accurately target sensitive sites in Saudi Arabia and the UAE, many hundreds of miles from Yemen's borders.

Their weapons now include cruise and ballistic missiles, some of which can fly more than 700 miles (1,200km), according to a recent report on the Houthis by Ms Katherine Zimmerman, a fellow at the American Enterprise Institute.

They have deployed pilotless kamikaze boats to strike ships in the Arabian Sea and have an array of drones that carry explosive charges and can fly as far as 1,300 miles.

Some equipment, like drone engines and GPS systems, are smuggled in with Iranian help, Ms Zimmerman wrote. But most of the group's weapons are made in Yemen. Drones are assembled from smuggled and local parts with Iranian technology and know-how, and missiles are built from scratch or modified to give them the range needed to reach deep inside Saudi Arabia.

So far, most Houthi attacks have caused limited damage, and their foes have learned to shoot down incoming drones and missiles.

But before the cease-fire began, Saudi Arabia often faced multiple attacks per month. The Saudi-led coalition said in December that the Houthis had launched 430 ballistic missiles and 851 armed drones at the kingdom since March 2015, killing 59 Saudi civilians.

And defending against incoming fire is hugely expensive. A missile for a Patriot defence system, for example, could cost US$1 million (S$1.36 million), Ms Zimmerman said, while Houthi drones and missiles are estimated to cost US$1,500 to US$10,000.

In a speech last month marking the seventh anniversary of the Saudi-led intervention, the Houthi leader, Mr Abdul-Malik al-Houthi, said the Saudi-led blockade of their territory and airstrikes on their bases and storehouses had pushed the group toward domestic weapons manufacturing.

The group's goal, he said, was to be able to strike any target, including in Saudi Arabia, the UAE or the Arabian Sea.

"We have worked to reach the level of launching from anywhere we want, even to the sea," said Mr al-Houthi. "We are very keen on that, to strike from any governorate to any point in the sea."