Britain has said it is sending more than 600 medical devices including oxygen concentrators and ventilators to India to help the country as it struggles to cope with a surge in coronavirus cases.

The equipment comes from Britain's surplus stock and the first shipment is due to arrive in New Delhi early on Tuesday, the British foreign ministry said.

"We stand side by side with India as a friend and partner during what is a deeply concerning time in the fight against Covid-19," British Prime Minister Boris Johnson said.