Mosquito-borne diseases including the Zika virus and dengue fever pose an "immediate threat" to survivors of the devastating earthquake that hit Ecuador four days ago, an international charity said on Thursday.

Efforts to help more than 24,000 people whose homes were destroyed or damaged by the 7.8 magnitude quake were hampered by heavy rain, and clean water remained one of the biggest needs, charity CARE International said.

"There is a lot of stagnant water which greatly increases the number of breeding sites for mosquitoes," Lucy Harman, CARE's emergency team leader in Ecuador, said in a statement.

"As a result, there's a higher risk of mosquito-borne disease such as Zika virus and dengue fever."

Large parts of Latin America and the Caribbean have been affected by the Zika outbreak, with Brazil the hardest hit so far.

In January Ecuador confirmed 22 cases of the virus, which the World Health Organization said was spreading "explosively" and could affect as many as four million people in the Americas.

The quake, the worst to hit Ecuador in decades, killed 570 people, injured 7,000, damaged nearly 2,000 buildings, and forced over 24,000 homeless survivors to seek refuge in shelters, according to government tallies.

Some isolated communities struggled without water, power or transport as torn-up roads stymied deliveries of aid.

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"Clean water is one of the biggest needs. People have made signs everywhere asking for water," Harman said.

CARE is distributing water purification tablets and supplying temporary water tanks, she said.

United Nations humanitarian chief Stephen O'Brien has called for more support for quake survivors who are living in the wreckage of their homes, exposed to the weather.

"Medical facilities and items are urgently needed for people who were injured, as well as food, clean water and basic sanitation," O'Brien said on Wednesday after a two-day visit to Ecuador.

The United Nations and its partners will launch an appeal on Friday for help for 350,000 people needing aid, he said.