MANILA - President Rodrigo Duterte has extended a sweeping, month-long lockdown over the northern third of the Philippines, where half the country's population live, as experts here have yet to ascertain the full impact of the coronavirus pandemic.

Cabinet Secretary Karlo Nograles said in a news briefing on Tuesday (April 7) that Mr Duterte had approved a recommendation by a task force coordinating efforts to roll back the viral spread to extend the "enhanced community quarantine" till April 30.

Mr Duterte placed the entire island of Luzon, which is the size of South Korea, on lockdown on March 16, and enforced some of the strictest measures in place in Asia, including curfews, checkpoints, work stoppage, and a halt to mass public transport. The capital Manila is on Luzon.

"We will most likely see the effect of the enhanced community quarantine in mid-April... That is why we are asking for an extension till April 30, so we can measure the full effect of the... (quarantine)," Mr Nograles, the task force's spokesman, told reporters.

He reiterated that it was still too early to tell if the Philippines was already "flattening the curve", three weeks into the Luzon-wide lockdown.

"We can't say yet if we've reached the peak, (the viral spread) slowed down or decelerated," he said.

The number of coronavirus cases in the country has surged in the past three weeks. From 187 on March 17, there are now 3,660 cases as of April 6.

Mr Nograles said health officials would have to be testing from 13,000 to 20,000 suspected cases a day to determine if the lockdown, which had resulted in massive job losses and crippled the economy, could be eased.

Test results would also have to come out quicker - within 24 hours - and virus carriers would have to be isolated faster, he said.

"If we can do all these things, experts are saying we can push the peak here in the Philippines to as late as 2021. By that time, we are hoping there will already be a vaccine available," he said.

"That's the game plan," he added. "Right now, we really can't see the peak."

Luzon has been effectively shut off from the rest of the Philippines and the world since mid-March. It is home to about 60 million and generates about 70 per cent of the country's gross domestic product (GDP).

All domestic flights and sea travel, except for those transporting essential goods, have been suspended.

Companies in Metro Manila and the rest of Luzon have wound down and sent their workers home, except for those providing food and medicine, utilities, banks, telcos and logistics firms.

A web of checkpoints and barricades across Luzon is making sure everyone stays put.

The government's economic planning agency has warned that at least a million workers could lose their jobs because of the lockdown.

The Asian Development Bank estimated that the country's GDP could plunge to 2 per cent this year from last year's 5.9 per cent. Cargo has been piling up unclaimed at Manila's port.

Mr Duterte has approved a stimulus package worth at least 200 billion pesos ($5.69 billion). Among other things, it will provide 5,000 pesos to 8,000 pesos in subsidies to 18 million low-income families.

But red tape has slowed disbursement of this aid, causing unrest.

Last week, residents of a slum north of Manila staged a protest along a highway near their shanty houses, claiming they had not received any food packs and other relief supplies since the lockdown began more than two weeks ago.

Police broke up the protest and arrested 20 people.

Mr Duterte later warned violators of his lockdown measures that they could be shot for causing trouble.

Mr Nograles said he expects the Philippines to transition to a "new normal" should the government decide to ease the lockdown.

"It won't be as restrictive, but social distancing still has to happen. Testing still has to happen," he said.

He said if a vaccine emerges next year, "then we can go back to the old normal".