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Kenyan President William Ruto put the flood-ravaged country on high alert on Friday and postponed the reopening of schools indefinitely as the nation braced for its first-ever cyclone.
Torrential rains have lashed East Africa since March and claimed the lives of more than 350 people.
The region is now threatened by a cyclone projected to make landfall at the weekend along its Indian Ocean coast.
"This cyclone named Hidaya, which could hit anytime now, is predicted to cause torrential rain, strong winds, and powerful and dangerous waves," Ruto told a press briefing in the capital, Nairobi.
"Our country must act swiftly and decisively to mitigate the devastating impacts of the present crisis and protect life and property."
Schools, which were due to reopen on Monday, will now remain shut indefinitely.
All ministers have been directed to coordinate the evacuation and relocation of all affected Kenyans.
Cyclone Hidaya will peak at gusts of 165 kilometres (100 miles) per hour when it makes landfall in neighboring Tanzania on Saturday, according to the Climate Prediction and Applications Centre for East African trade bloc IGAD.
Cyclone season in the southwest Indian Ocean normally lasts from November to April, and there are around a dozen storms each year.
Tanzanian authorities warned earlier on Friday that Hidaya had "strengthened to reach the status of a full-fledged cyclone" by 3:00 am (0000 GMT) when it was some 400 kilometres (250 miles) from the southeastern city of Mtwara.
"Cyclone Hidaya has continued to strengthen further, with wind speeds increasing to about 130 kilometres per hour," they said in a weather bulletin.
'No corner spared'
East Africa's rains have been amplified by the El Nino weather pattern -- a naturally occurring climate phenomenon typically associated with increased heat worldwide that leads to drought in some parts of the world and heavy downpours elsewhere.
So far, around 210 people have died in Kenya from flood-related incidents.
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More than 165,000 others have been uprooted from their homes and nearly 100 are missing, according to government data.
"No corner of our country has been spared from this havoc," Ruto said.
"Sadly, we have not seen the last of this perilous period," he warned.
At least 155 people have been killed in Tanzania by floods and landslides that have destroyed crops and swallowed homes.
Rescuers in boats and aircraft have raced against the clock in pouring rain to help people marooned by the floods in Kenya.
In dramatic footage shared on Wednesday, the Kenya Red Cross rescued a man who said he was stranded by floodwaters and forced to shelter in a tree for five days in Garissa in the east of the country.
The military also joined search and rescue efforts after Ruto deployed them to evacuate everyone living in flood-prone areas.
The government has ordered anyone living close to major rivers or near 178 "filled-up or near filled-up dams or water reservoirs" to evacuate the area within 24 hours.
Opposition politicians and lobby groups have accused the government of being unprepared and slow to respond to the crisis despite weather warnings.
Forced to escape again'
The heavier-than-usual rains have also claimed at least 29 lives in Burundi.
Some 175 people have been injured and tens of thousands displaced since September, the United Nations said.
UN refugee agency UNCHR said it was "particularly concerned" about thousands of refugees who had been displaced in Burundi, Kenya, Somalia and Tanzania.
"(They are) being forced to escape once again for their lives after their homes were washed away," UNHCR spokesperson Olga Sarrado Mur said on Friday.
Late last year, more than 300 people died in rains and floods in Ethiopia, Kenya, and Somalia, just as the region was trying to recover from its worst drought in four decades.