The United Nations and its partners on Thursday launched an appeal for $4 billion in aid this year for Yemen, devastated by nearly a decade of war and conflict.

The UN Office for the Coordination of Humanitarian Affairs (OCHA) said in a statement that its 2024 humanitarian response plan "requires $2.7 billion for live-saving assistance and protection services".

A further $1.3 billion was needed for sustainable development, the statement said.

The "urgent support" was needed for over 18.2 million civilians in Yemen, "who have faced tremendous suffering daily for more than nine years due to conflict, economic deterioration, severely disrupted public infrastructure and services, as well as climate change," said UN humanitarian coordinator Peter Hawkins.

"We must not turn our backs on the people of Yemen. I am appealing to donors for their continued and urgent support to save lives, build resilience, and also to fund sustainable interventions," he said.

The Arabian Peninsula's poorest country has been gripped by conflict since the Iran-backed Huthi rebels overran Sanaa in 2014, triggering a Saudi-led military intervention in support of the government the following year.

Hundreds of thousands of people have died directly from fighting or from indirect causes such as lack of food in what the UN has called one of the world's worst humanitarian crises.

OCHA said that "17.6 million people are estimated to face acute food insecurity in 2024."

Yemen "is experiencing some of the highest malnutrition rates ever recorded... and the situation continues to worsen," it said.

US downs drone off Yemen

American forces shot down a drone off Yemen on Thursday and later destroyed an explosives-laden uncrewed surface vessel that threatened ships in the Red Sea, the US military said.

The US Central Command (CENTCOM) also said Yemen's Iran-backed Huthi rebels launched two anti-ship missiles that were possibly aimed at a cargo ship in the Red Sea, but that the missiles did not hit the vessel.

There has been a sharp uptick in Huthi military activity and US strikes and shootdowns in recent days, presenting further challenges for Washington as it prepares what it has promised will be a decisive response to the killing of three American soldiers in a drone attack in Jordan over the weekend.

Both Washington and Tehran have sought to avoid an all-out war, and the United States aims to deter further attacks but not trigger direct conflict with Iran, while also contending with other violence in the region.

CENTCOM said US forces shot down a drone over the Gulf of Aden early on Thursday, without identifying who it belonged to, while it blamed the Huthis for the bomb-rigged surface vessel, the destruction of which resulted in "significant secondary explosions" but no reports of damage or injuries.

The Huthis later fired two anti-ship ballistic missiles, "likely towards the M/V Koi in the Red Sea. The missiles impacted in the water without hitting the ship," CENTCOM said, noting that the vessel is Liberian-flagged and Bermuda-owned.

- 10 Huthi drones destroyed -

The United States also destroyed 10 Huthi drones and a ground control station shortly after midnight local time on Thursday, while an American warship shot down a missile fired by the rebels as well as three Iranian drones the previous day.

The Huthis began targeting Red Sea shipping in November, saying they were hitting Israel-linked vessels in support of Palestinians in Gaza, which has been ravaged by the Israel-Hamas war.

US and UK forces have responded with strikes against the Huthis, who have since declared American and British interests to be legitimate targets as well.

On Thursday, Huthi military spokesman Yahya Saree said on social media that the rebels targeted a British commercial ship in the Red Sea "in victory for the oppression of the Palestinian people and as part of a response to the American-British aggression against our country."

That incident could not immediately be independently confirmed.

In addition to strikes against the Huthis, the United States set up a multinational naval task force aimed at protecting shipping on the transit route, which carries up to 12 percent of global trade.

Anger over Israel's devastating campaign in Gaza -- which began after an unprecedented Hamas attack on October 7 -- has grown across the Middle East, stoking violence involving Iran-backed groups in Lebanon, Iraq, Syria and Yemen.