Aid agencies have warned of a growing humanitarian crisis in Yemen as the UN says the majority of people killed in the conflict are civilians, blaming both the Saudi-led coalition and Houthi rebels. 

"Over 600 people [have been] killed [in the conflict], but more than half of them are civilians. This is particularly concerning," Ivan Simonovic, UN's deputy secretary-general for human rights, told Al Jazeera on Monday. 

"So far we can say with confidence that both sides have not exercised sufficient restraint. There were some unselective targeting and we are very concerned about that."

Simonovic said it was essential not to allow "the acute crisis evolve into a chronic one".

"There is still a window of opportunity when fighting and killing in Yemen could be stopped," he said.

Nine Arab countries led by Saudi Arabia launched air strikes on Shia rebels on March 26 after the rebels stormed the presidential palace in the capital Sanaa and put President Abd-Rabbu Mansour Hadi under house arrest, demanding political reforms.

The rebels, known as Houthis, swept into Sanaa in September and have since tried to expand their control across the country. They are fighting army units loyal to Hadi, who fled to Saudi Arabia, and are backed by security forces supporting Ali Abdullah Saleh, Yemen's former president.

The coalition is supported by the United States, which has supplied arms and has also carried out drone attacks against al-Qaeda fighters in Yemen.

Overnight on Monday, Yemen's main southern city of Aden saw the heaviest fighting, with medics and military forces saying at least 30 people were killed in clashes between rebels and supporters of Hadi.

Humanitarian groups have struggled to bring aid into the country and said on Monday the situation in Aden was deteriorating rapidly.

"Shops are closed. We have a problem of food," said Marie-Elisabeth Ingres, the Yemen representative of Doctors without Borders (MSF).

Metaz al-Maisuri, an activist living in Aden, said basic services had stopped and there had been a "mass exodus" of civilians from the city.

"Schools, universities and all public and private facilities have been shut due" to the violence, he told the AFP news agency. "Residents' lives have become very difficult and complicated... They can no longer obtain the food they need," he said.

"We are unable to leave our houses to buy what we need because of the Houthi snipers," said Adwaa Mubarak, a 48-year-old woman in Aden.

'Boots on the ground needed'

Afzal Ashraf, a consultant fellow at the London-based Royal United Services Institute, told Al Jazeera that the Saudi-led coalition faced a dilemma over getting their military on the ground as air strikes alone would not achieve the coalition's aims.

"The situation is very confused not just for us, observers, but also for people on the ground. And it will remain that way until we get ground forces in," said Ashraf.

"This is the problem that the Saudi-led coalition is facing. They want to avoid ground forces, but they can't make any meaningful change on the ground using air strikes alone."

Meanwhile, in the Saudi capital Riyadh, Yemen's Prime Minister Khaled Bahah was sworn in as vice president at the country's embassy in front of exiled Hadi, a day after his appointment, in a move welcomed by Yemen's Gulf neighbours.

Mohammed Abdel Salam, a Houthi spokesman, denounced the appointment of Bahah in televised comments on a pro-Houthi channel. He said that the Houthi group will not recognise decisions promulgated by Hadi and that anything pertaining to the country's politics should be decided upon through dialogue within the country.

UN special envoy for Yemen Jamal Benomar has been urging the parties to come to a negotiated settlement. Saleh has also called for a UN-sponsored dialogue.

Source: Al Jazeera and agencies