More than half of the 4,000 Indian nationals caught up in the conflict in Yemen are nurses, the foreign ministry said on Monday, the latest security scare for medical staff working in the Middle East after dozens were kidnapped in Iraq last year.

The nurses, mostly from the southern state of Kerala, are often hired on harsh terms with middlemen taking up-front fees, and hospitals are reluctant to let them go because they would have to close without foreign staff.

Sajeesh Mathew's wife, 29-year-old nurse Asha, has worked for three years at the Al-Naqib Hospital in the port city of Aden, scene of heavy fighting on Monday.

"The areas around the hospital are now under the control of the Houthi rebels," said Mathew, whose wife is one of 35 Indian nurses at the hospital.

The city's port is destroyed and the road to Sanaa is unsafe, he added.

Although no Indian casualties have been reported, the nurses' predicament in Yemen recalls the ordeal suffered by 46 Indian nurses kidnapped from a hospital in Iraq last year as Islamic State militants advanced on Tikrit.

The nurses were freed and evacuated in June, in an early diplomatic triumph for Prime Minister Narendra Modi's new government, but the fate of 39 Indian building workers taken captive in Mosul remains unclear.

India's Ministry of External Affairs (MEA) is mounting an attempt to airlift nationals from the Yemeni city of Sanaa.

Eighty Indians were flown out on Sunday to Djibouti, on the opposite shore of the Gulf of Aden.

Two Air India aircraft were on standby in Muscat, Oman, but had not been granted permission to fly to Sanaa to evacuate Indian nationals, MEA spokesman Syed Akbaruddin told reporters on Monday.

He said 400 Indians were being evacuated by sea from Aden and would reach Djibouti on Tuesday. They will be flown home by the Indian Air Force.

New Delhi has issued a series of warnings this year to Indian nationals to leave Yemen, the last of them shortly before Saudi Arabia launched air strikes last Thursday against Iranian-allied Houthi militiamen.

Ruben Jacob Chandy heeded the call, taking a scheduled flight out of Sanaa and arriving back in the Kerala capital Thiruvananthapuram on Monday, with a handful of other Indians who escaped the fighting.

"The situation is critical," said Chandy. "The Saudis are carrying out a lot of air targeting - it starts from 6pm until almost 6am."

An Indian navy patrol vessel involved in anti-piracy operations was heading for Aden, and would be joined by two more navy ships. Two passenger ships with the capacity to carry 1,100 people had also set sail from India, Akbaruddin said.

Indians returning from Yemen said the situation, especially in Aden, was grave.

"They cannot go out of their residences. Many are running out of water and food. There is no way they can go out and procure the essentials," said Lijo George, an IT worker who returned to Kerala on Monday from Sanaa.

Speaking from the Military Hospital in Sanaa, paramedic Ranjith Cheerakathil said he and his wife, a nurse, were among the few who had decided to stay. Most of the hospital's 240 Indian staff were waiting for a flight to leave.

"Most of the operations in the hospital will be shut down when they leave. There will not be anybody to care for those who suffer injuries in the attack," Cheerakathil said by telephone.

"This is cruel. My conscience does not allow me to leave them like that," he added.-Reuters