A top surgeon in New York City has warned that the coronavirus outbreak has "breached" hospital walls as the city's stretched-thin health system braces for a "peak" number of infections within the next few weeks.

In a series of daily memos about the state of the city's healthcare capacity, the surgeon-in-chief at New York-Presbyterian Hospital/Columbia University Irving Medical Center and the chair of Columbia University's Department of Surgery has projected that the hospital system could be overwhelmed by Covid-19 within 30 days.

According to Dr Craig Smith, by then New York-Presbyterian would need up to 934 intensive care unit beds but won't realistically have even 700.

In a memo to colleagues on Friday, he said: "The hard data has become alarming. I wish I could use a more comforting word."

On Saturday, he said he anticipates an "increasing number of infected Department colleagues" as infections begin appearing in "clusters associated with the care of infected patients". He predicts that the "reservoir of undetected" people unknowingly infected with the virus is "much larger than we imagined" and will become apparent in the coming days.

The warnings follow calls to retired nurses and doctors to join a reserve of volunteer health workers to boost the city's overrun health system.

Within one day last week, 1,000 retired health workers signed up with the city's Medical Reserve Corps under the Department of Health at the urging of Mayor Bill de Blasio.

Those ranks join roughly 9,000 other medical volunteers to sign up with the department.

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More than 4,500 cases of the virus have been confirmed in the city itself, with statewide patients edging past 11,600. As of 22 March, at least 60 people in the city have died.

The number of cases across the US has reached nearly 25,000 with more than 300 deaths reported.

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New York Governor Andrew Cuomo has joined governors in New Jersey and Connecticut to urge tri-state residents to stay indoors, and Governor Cuomo has ordered the closure of non-essential businesses statewide.