India has accelerated efforts to relocate more than a million people along its northeast coastline as a cyclone intensifies ahead of its expected landfall on Friday.

In total, about 1.2 million people are expected to flee the low-lying areas of 15 districts in the eastern state of Odisha — to cyclone shelters, schools and other buildings, authorities said. More than 800,000 people have left so far.

"We are maximizing efforts at all levels for evacuation," said Odisha's special relief commissioner Bishnupada Sethi.

Cyclone Fani is forecast to make landfall on Friday with winds of up to 200 km/h, according to the state-run India Meteorological Department (IMD).

On Thursday, the storm was churning up the Bay of Bengal, about 275 kilometres south-southwest of the Hindu temple town of Puri, with sustained winds of 170-180 km/h. Cyclone tracker Tropical Storm Risk rated it as a Category 4 storm, a notch below the worst level.

The IMD warned of "extremely heavy falls" over parts of the state of Odisha and its southern neighbour, Andhra Pradesh.

India's National Disaster Management Authority forecast "high to phenomenal" sea conditions for most of the Indian states along the Bay of Bengal. Fishermen were advised not to venture into deep waters. A 1.5-metre storm surge was expected to inundate low-lying areas.

Fearing that Fani could be the worst storm since 1999, when a cyclone killed some 10,000 people and devastated large parts of Odisha, Indian officials put the navy, air force, army and coast guard on high alert, the Press Trust of India news agency reported. The navy has deployed seven warships and has six planes and seven helicopters on standby along with divers, rubber boats, medical teams and relief materials.

Prime Minister Narendra Modi tweeted that the federal government will provide all possible assistance.

The Meteorological Department projected the "total destruction" of thatched-roof huts, flooding of farmland and orchards, and the bending and uprooting of telephone poles.

Authorities have also shut down operations at Paradip, Dhamra and Visakhapatnam ports.

In Paradip, television footage showed residents piling bicycles, sewing machines and gas cylinders onto small trucks and leaving for any of the nearly 900 shelters supplied with food, water and medicine.

Odisha state government has deployed hundreds of disaster management personnel, closed schools and colleges and asked doctors and other health officials not to go on leave until May 15.

"We've been preparing plans for the last few days to ensure that all the people who are vulnerable will be shifted to our cyclone centres," said Sethi, the special relief commissioner .

The National Disaster Response Force dispatched 54 rescue and relief teams to flood-prone areas along the coast and as far afield as Andaman and Nicobar Islands, a group of islands that comprise a state about 1,300 kilometres east of mainland India in the Bay of Bengal.

The teams included doctors, engineers and deep-sea divers equipped with boats, scuba sets and satellite phones, the group said in a statement.

In the coastal city of Srikakulam in Andhra Pradesh, strong winds and heavy rain battered an empty beach where fishing boats had been left on shore. People piled into truck beds and buses reinforced with tin sheeting.

Residents tied plastic tarps over their tin-roof shacks before abandoning them.

Fani was also forecast to hit Bangladesh, tracking north through ports including Cox's Bazar, the coastal district where more than a million Rohingya from Myanmar live in refugee camps.

Aid agencies warned the Rohingya were at threat. Hillol Sobhan, local communications director for the group NGO Care, said it is keeping emergency supplies for the refugees in Cox's Bazar.

The Bangladesh Inland Water Transport Authority said it suspended operations of all vessels. Authorities also halted activities at Chittagong Seaport, which handles 80 per cent of the country's overseas trade.

In a tweet, Indian airline Vistara said it would waive cancellation charges for flights to Odisha's capital, Bhubaneswar, and Kolkata, the capital of West Bengal, until Sunday.

Kolkata airport will be shut down from 11:30 p.m. local time on Friday until 6 p.m. on Saturday, while Bhubaneswar Airport will be shut on Friday.

IndiGo, the country's largest domestic air carrier, said it had cancelled flights to and from Bhubaneswar on Friday. Another airline, GoAir, said in a tweet that it would waive cancellation charges for flights to Bhubaneswar, Kolkata and Ranchi for travel until May 5.

Indian Oil Corporation, the country's top refiner, said its 300,000-barrels-per-day Paradip refinery in Odisha state did not need to shut down for now.

An executive at Reliance Industries, which operates an oil and gas block off the east coast, said its operations had not been affected. India's National Aluminium Company said there was no need to halt operations.

State-run power company NTPC has no plans to shut the 3,000-megawatt Talcher power plant in Odisha, its head of operations, Prakash Tiwari, said.