Shia rebels fighting for the control of Yemen and forces backing President Abd-Rabbu Mansour Hadi have engaged in fierce clashes in the country's south, leaving more than 100 people dead in 24 hours, as the Red Cross faces delays to deliver vital supplies.

Monday's clashes happened in Aden, Yemen's third-largest city and a power base for Hadi who fled to Saudi Arabia as the rebels, known as Houthis, expanded their control across the country.

Citing unnamed officials, the AFP news agency reported that 17 civilians were among 114 killed in Aden, where fighting continued as rebels tried to seize a port in the city.

The clashes came amid reports that Hadi had sacked three of his top military officers. The officers include General Abdullah Khayran, the Chief of Staff; Deputy Chief of Staff General Zakariah al Shami and Head of Sepcial forces, General Abdularrazak Almrouni.

Relief workers have warned of a dire situation in the impoverished Arabian Peninsula state, where a Saudi-led coalition is waging an air war on the Iran-backed rebels.

The International Committee for the Red Cross said that despite getting clearance from the coalition, an aid plane it dispatched was grounded in Djibouti and not allowed to fly.

Sitara Jabeen, the ICRC's spokesperson, told Al Jazeera that the humanitarian situation was worsening.

"The situation in Yemen remains extremely critical. The conflict ... has intensified, especially in Aden. We are still trying to find a cargo plane that can carry our supplies to Sanaa," said Jabeen, speaking from Sanaa, the Yemeni capital.

"We got permission from the coalition yesterday [to deliver supplies], but so far we have not been able to find a logistical solution to this problem ...There are less and less airlines that are flying to Sanaa and the country’s airline itself has suspended flights until further notice."

Houthi resistance

The Saudi-led coalition has bombed Houthi positions since March 26 and has dropped weapons to Hadi loyalists, but the rebels continue to put up resistance and have said they will accept peace talks only if the aerial attacks stop.

The total number of people killed since the air strikes began is not known, but relief agencies say more than 330,000 people have been displaced. Many more in Aden and Sanaa are in need of basic supplies.

Basharaheel Hisham Basharaheel, deputy editor of Al-Ayyam newspaper, told Al Jazeera: "People are running out of food. There is no water. No power supply … The hospitals are in a much worse shape. We see a lot of injured people with no means of saving them. No basic first aid kits, for example."

At least three Red Crescent volunteers were killed over the past week while evacuating wounded and retrieving dead bodies from the fighting in Aden and in the southern province of al-Dhale, reported the Associated Press news agency. ICRC called the killings deliberate in a statement on Friday.

"There are dead bodies on the streets in Aden. This is why we called for 24-hour humanitarian pause in the fighting so that people can also go and collect the dead," the ICRC's Mari-Claire Feghali said.

The Houthis swept into Sanaa in September and put Hadi under house arrest before he fled to Aden and then to Saudi Arabia. Backed by militias loyal to former president Ali Abdullah Saleh, they control large swaths of Yemen, which is also grappling with al-Qaeda.

Al Jazeera