Pro-government militiamen and Houthi rebels have committed serious abuses against detainees in Yemen's second city Aden, Human Rights Watch says.

The US-based group alleges southern militias have summarily killed at least seven Houthi prisoners since March.

In one case, a rebel was beaten in a public square before being shot dead.

The rebels are accused of unlawfully detaining and mistreating civilians before they were driven out of Aden in July after months of fierce fighting.

Southern fighters, backed by a Saudi-led coalition seeking to restore Yemen's exiled president, have advanced northwards since then, creating a security vacuum that has reportedly been filled by jihadist militants from al-Qaeda and Islamic State (IS).

Since Sunday, a senior security official and two leaders of the so-called Southern Resistance, an alliance of groups including secessionists seeking an independent South Yemen and army units loyal to President Abdrabbuh Mansour Hadi, have been shot dead by gunmen on motorbikes in Aden.

No group has said it was behind the killings, but al-Qaeda's local offshoot has used motorbikes in previous attacks.

Prisoner boat 'blown up'

Human Rights Watch (HRW) said on Tuesday that it had documented several cases of serious abuse committed by southern militiamen and Houthis against civilians and fighters in their custody since the battle for Aden began in late March.

On 23 August, southern militiamen are alleged to have placed an unidentified group of Houthi prisoners in orange jumpsuits on a boat in the middle of the port of Aden and then blown the boat up.

They reportedly filmed the explosion to the backdrop of IS flags raised on the port buildings.

Why is there fighting in Yemen?

• Northern Shia Muslim rebels known as Houthis, backed by forces loyal to Yemen's ex-president, took over parts of Yemen, including the capital, Sanaa, and forced the government into exile in March
• The rebels accused the government of corruption and of planning to marginalise their heartland within a proposed federal system
• Forces loyal to the government and southern militias are fighting back, aided by air strikes led by neighbouring Saudi Arabia

Yemen's humanitarian catastrophe

Who is fighting whom?

Meeting the Houthis and their enemies

The next day, a Houthi prisoner was taken to a public square and beaten by a dozen southern fighters, who encouraged bystanders to join in, before shooting and killing him, a witness told HRW.

HRW also said two wounded Houthi officers, who were patients at a hospital in Aden, had been summarily killed in March. Three other Houthis were removed at the time and shot, hospital staff reported.

Houthi fighters are meanwhile accused of having threatened captured southern fighters and held them in harsh conditions before withdrawing from Aden.

They also detained civilians, including aid workers, and took the medical supplies and other goods they were transporting, according to HRW.

HRW called on Yemen's exiled government and the United Arab Emirates, which has reportedly landed several thousand troops in Aden in recent weeks, to press the authorities in Aden to end abuses and punish those responsible.

"Southern forces that have regained control of Aden should end abuses against prisoners and do all they can to establish law and order in the city," said Sarah Leah Whitson, the group's Middle East director.

The UN says some 4,500 people - including at least 2,112 civilians - have been killed since 26 March, when the Saudi-led coalition began an air campaign to defeat the Houthis and restore President Hadi.