Pro-Houthi forces shot and killed two women and held aid workers hostage in Yemen's southern city of Aden, a rights group has said, hinting at possible war crimes.

The New York-based Human Rights Watch said on Thursday the incidents, which happened last month, exemplify the grave threats to civilians in the embattled port city where Shia rebels and their allies are fighting forces loyal to President Abd-Rabbu Mansour Hadi. 

HRW said the two women were struck by gunfire in separate incidents on April 17 and 18, and died before relatives could find a medical facility to treat them. 

Pro-Houthi forces also unlawfully detained 10 local aid workers for 14 days, releasing two only after payments were made. Deliberate attacks on civilians and taking hostages are war crimes, the group said. 

HRW identified the victims as Sabreen al-Aboos, 20, and Neveen al-Taib, 42. 

The accusations came a day after a boat carrying civilians attempting to leave Aden was shelled, killing 40 civilians. 

Yemen's Defence Minister Riyadh Yassine, speaking in the Saudi capital Riyadh, pointed the accusing finger at the Houthi rebels. 

At least 120 people have been killed in Aden since fighting began there in March but some of the victims have been killed in air strikes launched by a coalition of Arab nations led by Saudi Arabia. 

Saudi casualties 

Meanwhile, shells fired from Yemen killed five people in Saudi Arabia on Wednesday in a second day of cross-border bombing blamed on the Houthi rebels, civil defence authorities said. 

Two civilians in a car and two passers-by were killed by a shell in the city of Najran, while 11 others were wounded, the civil defence department said in a statement carried by the SPA state news agency. 

A prison security officer was also killed and another wounded when a second shell landed on their patrol in the same area, the statement said.

Despite weeks of aerial bombardment against Houthi positions, the conflict shows no sign of ending.

Yemen's mission to the United Nations on Wednesday called for a ground intervention to push back the rebels' advance in Aden. 

"We urge the international community to quickly intervene by land forces to save Yemen, especially Aden and Taiz," Khaled Alyemany, Yemen's Ambassador to the UN, said in a letter to the Security Council. 

The letter obtained by Al Jazeera also called on the international human rights organisations to document "barbaric violations against a defenceless population". 

'Concerns' 

Al Jazeera's Mohammed Vall, reporting from Saudi Arabia, said the choices for the Saudis are difficult as deploying ground forces means "more casualties". 

The Saudis have a "lot of concerns" following rebel attacks that have killed Saudi soldiers and civilians, said Vall.

Houthi fighters - widely believed to be backed by Shia Iran - swept into Sanaa in September and have since tried to expand their control across Yemen, which is also battling al-Qaeda's local branch, Al-Qaeda in the Arabian Peninsula (AQAP). 

The Houthis put President Hadi under house arrest in February, demanding he carry out political reforms. 

Hadi has since fled to Sunni-majority Saudi Arabia, which says it launched the air strikes against the Houthis to help defend a "legitimate authority" and has rejected Iran's calls to halt the bombardment.

Source: Al Jazeera and agencies