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Rebel forces in Yemen have fired shells and rockets indiscriminately into civilian neighbourhoods in the southern port city of Aden, Human Rights Watch alleged on Thursday.

The watchdog said anti-government forces including Houthi rebels had acted "in violation of the laws of war" and called on them to "immediately cease indiscriminate attacks" on populated areas.

"Pro-Houthi forces have been raining mortar shells and rockets on to populated areas of Aden with no apparent regard for the civilians remaining there," HRW's senior emergencies researcher Ole Solvang said.

"These unlawful attacks take a terrible human toll and should stop immediately."

The rights group said it had identified one rebel attack, in Aden on July 19, that killed several dozen civilians including children.

The human rights group has also criticised the actions of the Saudi-led coalition, whose bombing has left civilians dead across the country.

Houthi rebels and allied forces loyal to former Yemeni president Ali Abdullah Saleh have been battling local pitted against them, as well as pro-government forces allied to them, in and around Aden for months.

The anti-Houthi forces, many of whom are secessionists demanding the return of the former South Yemen, finally pushed the Houthis out of most of the city earlier this month, with the help of Saudi-led coalition warplanes.

A nationwide pause in fighting this week proposed by the coalition failed to take hold and threatened the delivery of vital aid to people affected by the violence.

Four months of fighting have left almost 4,000 people dead, nearly half of them civilians, according to the United Nations.

"Houthi leaders should realise that they could face a war crimes trial for ordering or even just overseeing indiscriminate rocket attacks on civilian neighbourhoods," Solvang said.

US drone attack kills 5

Meanwhile, in the south of the country, a suspected US drone strike killed five presumed members of al-Qaeda, a local official said on Thursday.

A vehicle carrying the men was hit overnight as it travelled through Wadi Dikha in Abyan province, the official said, after it had set off from the southern city of Mukalla, where al-Qaeda militants have been able to operate freely.

Five people, including a local al-Qaeda chief named as Abu Ahmad al-Kazimi, died in the strike, according to the official.

The United States is the only country known to operate armed drones over Yemen, and attacks by the unmanned aircraft have continued during the ongoing war in the country.

Al-Qaeda in the Arabian Peninsula (AQAP), considered by Washington to be the extremist network's most dangerous franchise, announced the death in June of its Yemen commander Nasir al-Wuhayshi in a US drone attack.

AQAP was behind several plots against Western targets and claimed the January massacre at French satirical magazine Charlie Hebdo.

The group has taken advantage of the chaos gripping Yemen to make territorial gains in the south, including stengthening its hold of Mukalla, the capital of the vast desert Hadramawt province.

'Islamic State' car bomb outside Sanaa mosque

The fighting in Yemen has also given the opportunity for Islamic State group (IS, formerly ISIS) operatives to operate in the country, and the group claimed a car bomb that exploded outside an Ismaili Shia mosque in Yemen's capital on Wednesday, killing four people.

Witnesses said the blast was near a mosque of the Bohra Ismaili sect of Shia Islam, not far from Ath-Thawra Hospital in the Rammah district of Sanaa, where several attacks claimed by IS have targeted Houthi rebels who control the capital.

An IS Twitter user calling himself Karar al-Moayad said the militant group had carried out the attack and that it had left "several dead," while a medical source said another four people were wounded.

Four people were also killed in a similar explosion on July 21, according to a toll provided by the Houthis.

Bomb attacks by the extreme Sunni IS, which considers Shias to be heretics, targeted several mosques in the capital on March 21 and killed 142 people.

Most of the mosques were associated with the Houthis, but mosques in Sanaa traditionally operate without overt sectarian affiliation.

Meanwhile, clashes raged in the south, where anti-Houthi forces expanded their area of control after recapturing the port of Aden, Yemen's second city, after four months of fighting.

The fighters pushed back the Houthi-Saleh forces in Lahoum, on Aden's northern outskirts, following heavy fighting in which the Houthi-Saleh fighters lost 12 men, military sources said.

Three anti-Houthi fighters were killed and dozens wounded, medical sources said.

The area is on the road to Lahj, where anti-Houthi fighters, labelled the Popular Resistance, have been tightening the noose on the Houthis, aiming to recapture the strategic Al-Anad airbase.