Spain’s maritime rescue service has found the bodies of 11 people, among them a pregnant woman, on a small boat off the country’s southern coast.

The discovery came at the end of a two-day search for the migrant boat and its 55 passengers, who had gone missing about 11 miles (18km) south of the city of Almería.

A spokeswoman for the rescue service told the Associated Press that a Spanish navy scientific research vessel had found the drifting boat in the area, east of the strait of Gibraltar, in the early hours of Thursday.

Twenty-nine men and four women were still alive onboard, but one of the men later died in a hospital in Almería. Another 11 people remained missing.

The service has also been looking for three more boats in the Alboran Sea, two of them carrying more than 55 migrants each.

The Almería branch of the Spanish Red Cross called it a “sad and tragic day”, adding that the survivors and the bodies of the dead had been brought into the city’s port.

According to local government officials, three young men were still under observation in hospital.

Figures from the International Organization for Migration showed more than 55,200 migrants and refugees had reached Spain by sea so far this year, while 743 died in the attempt.

“Already this month, more men, women and children have arrived via sea from North Africa than did in any of the year’s first four months,” the IOM said in a recent bulletin.

“Migrant sea arrivals to Spain have exceeded 150 per day since 1 November. For the first third of 2018, Spain’s average was under 40 arrivals per day.”

The record number of arrivals to Spain – partly driven by the closure of other European routes – has placed huge strains on the country’s migrant reception infrastructure.