Colombia faces its worst natural disaster on record due to the effects of relentless heavy rain that has been pounding most of Colombia this year, President Juan Manuel Santos said late Monday.

The heavy rain, triggered by the La Nina weather phenomenon, has killed at least 69 people in April alone, bringing the total death toll to at least 90, officials said.

"It's as if our territory had been struck by a hurricane that arrived last year and does not want to leave," Santos said in an address to the nation.

"This is without doubt the worst natural tragedy of our history," he said, as he called for "national unity" to face the disaster.

The heavy rain has caused damage in 28 of the country's 32 departments, and has blocked 16 major roads due to landslides. Some have collapsed entirely.

Three million people have been affected by the heavy rains since 2010, Santos said.

This month at least 36 people have been wounded and eight are missing, while another 98,000 were injured and 183 homes were destroyed by effects of the rain.

That brings the total this year to 15 missing and 208,581 people affected by the disaster, according to the Interior and Justice Ministry. In 2010 more than 300 people were killed by the effects of heavy rain.

Santos said that work scheduled to alleviate the effects of the first round of heavy rain in 2010 has not proceeded because there has been no pause in the rain, so the government has focused instead on emergency aid.

Police are focusing on rescue, evacuation, food distribution, security at shelters, manning state roads and other emergencies, according to a statement from the police directorate.

Some 160,000 Colombian police officers and 52 aircraft are participating in emergency operations.

More than 900,000 hectares (2.22 million acres) of land have been ravaged by rising rivers that overflowed, according to government figures. The Colombian government has provided victims $176 million in aid.