Coronavirus-linked flight restrictions are hampering efforts to wipe out locust swarms on the verge of devastating crops in East Africa, according to the United Nations Food and Agriculture Organization (FAO).

The curbs have delayed deliveries of pesticides and, at the current rate of spraying, stocks in Kenya will run out within four days, Cyril Ferrand, FAO's head of resilience for Eastern Africa, told Reuters news agency on Friday.

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"If we fail in the current [regional] control operations, because of lack of pesticides, then we could see four million more people struggle to feed their families," Ferrand said.

Locust numbers exploded late last year, encouraged by unusual weather patterns amplified by climate change, and swarms disbursed eastwards from Yemen, with Kenya, Somalia and Ethiopia the hardest-hit countries.

The first invasion that hit farmers in a region where millions of people struggle for food has given birth to a second wave of insects just as new-season crops are being planted.

"They are very active, very voracious, and very mobile," Ferrand said. "If we don't have pesticides, our planes cannot fly and people cannot spray and if we are not able to control these swarms, we will have big damage to crops."

In Kenya, the FAO was now looking to secure pesticides from local sources, Ferrand said, should the delays continue.

The spreading of the new coronavirus has forced governments to close their borders, reducing cargo flights and disrupting global supply chains, including the production of pesticides in Europe and Asia.

An order of pesticides due to arrive in Somalia by the end of last month has been delayed, though Ethiopia managed to secure enough of the chemicals before cargo flights were cut back.

Also, a lockdown imposed in South Africa last week has made it difficult to secure the helicopters that are crucial for locust surveillance.

"We need to have mobility that is equivalent to the desert locusts, that's what helicopters give us," Ferrand said.

The FAO has secured about $111m of funding towards fighting the swarms. But that is $40m less than the organisation sought and contributions have dropped off since mid-March, Ferrand said.

Humanitarian corridors

Meanwhile, aid organisations are negotiating for humanitarian corridors in peaceful regions after at least 32 countries closed their borders, according to Africa Centres for Disease Control and Prevention data.

Some nations have made exceptions for cargo, humanitarian aid or emergency flights.

But at least 21 low-income and middle-income countries, most of them in Africa, are already seeing shortages of vaccines because of border closures or flight disruptions, Gavi, the Vaccine Alliance, said in a statement on Friday.

The UN children's agency earlier noted "major disruptions" of vaccine shipments by air, notably to West Africa, as many originate in Europe, and some countries have refused to accept shipments from nations with virus outbreaks.

"If the chaos caused by this pandemic is allowed to curtail humanitarian assistance, the results will be catastrophic," the medical charity Doctors Without Borders (Medecins Sans Frontieres, or MSF) warned in a statement on Friday.