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Yemen's insurgents carried out twice as many attacks onÌýSaudi territory from January to September this year as they did in the same time periodÌýlast year, an American think tank said on Tuesday.

Research from the (CSIS) found the rebels have been "orchestrating an increasingly intense irregular warfare campaign against Saudi Arabia and other countries in the Gulf".

The HouthisÌýhaveÌýbeen aided by the Ìý-Ìýa special operations and intelligence arm of Iran's military -ÌýandÌý Shia political party and militia , according to the CSIS.

These organisations have given the Houthis weaponry and instructed them on how to use it, the think tank said.

Iran and the Houthis have strongly denied claims of military collaboration.

The rebels have recently been deploying more sophisticated technologyÌýincludingÌýcruise and ballistic missiles,Ìýaccording to the CSIS, many of which have been directed at Saudi Arabia, which is leading an Arab coalition against the Houthis.

"The Houthis' recent ability to ramp up attacks indicate that they pose an enduring threat to Saudi Arabia,"Ìýthe think tank said.

Between 1 January and 30 September last year, the rebels carried out 14 attacks in Saudi territory. They carried out 33 attacks in the first nine months of 2021.

The CSIS said the locations attacked by the Houthis in Saudi Arabia were "predominantly civilian".

Over 80 percent of the rebels' attacks have been in Saudi Arabia's Asir, Jazan, and Najran regions, which border Yemen.

Attacks had been decreasing since a spike in 2018, until this year.

The war in Yemen began in 2014, when the Houthis seized vast swathes of the nation and took its capital Sanaa.

"It has been three years since the Stockholm Agreement, with no progress on the handing over of Hodeidah port by the Houthis, the opening of roads into Taiz city, or a comprehensive prisoner exchange." on the 'new phase' in the war in Yemen ⬇

— °®Âþµº (@The_NewArab)

Saudi Arabia and its allies intervened in 2015, seeking to restore Yemen's government.

The years of war that followed have createdÌý "the world's worst humanitarian crisis", the United Nations has said.

The UN estimated that 377,000 people will have perished due to the war before 2021 is over.

Thousands are in Saudi airstrikes in Yemen.

It said that around 30 million people, or almost 80 percent of the Yemeni population, require aid and protection. More thanÌý13 million are at risk of starving.