Libyan authorities have blocked civilians from entering the flood-stricken eastern city of Derna to allow search teams to look through mud and wrecked buildings for 10,100 missing people, as the known death toll rose to 11,300.

The disaster after two dams collapsed in heavy rains and sent a massive flood gushing into the Mediterranean city early on Monday underlined both the storm’s intensity and Libya’s vulnerability.

Since 2014, the oil-rich state has been split between rival governments in the east and west backed by various militia forces and international patrons.

Derna is being evacuated and only search and rescue teams would be allowed to enter, according to eastern Libya’s emergency services director-general Salam al-Fergany.

The disaster has brought rare unity, as government agencies across Libya’s divide rushed to help the affected areas, with the first aid convoys arriving in Derna on Tuesday evening.

Relief efforts have been slowed by the destruction after several bridges that connect the city were destroyed.

The Libyan Red Crescent said that as of Thursday that 11,300 people in Derna had died and another 10,100 were reported missing. Mediterranean storm Daniel also killed about 170 people elsewhere in the country.

Eastern Libya’s health minister, Othman Abduljaleel, has said the burials so far were in mass graves outside Derna and nearby towns and cities.

Mr Abduljaleel said rescue teams were searching wrecked buildings in the city centre and divers were combing the sea off Derna.

Soon after the storm hit the city on Sunday night, residents said they heard loud explosions when the dams outside the city collapsed.

Floodwaters gushed down Wadi Derna, a valley that cuts through the city, crashing through buildings and washing people out to sea.

Lori Hieber Girardet, the head of the risk knowledge branch the UN Office for Disaster Risk Reduction, told The Associated Press on Thursday that because of years of chaos and conflict Libyan “government institutions are not functioning as they should”.

As a result, she said, “the amount of attention that should be paid to disaster management, to disaster risk management, isn’t adequate”.

The city of Derna is governed by Libya’s eastern administration, which is backed by the powerful military commander Khalifa Hiftar.