The South Korean and United States militaries announced on Thursday they were postponing their annual joint drills amid concerns about the novel coronavirus outbreak, which has infected 1,766 people in the Asian country, the largest outbreak outside China.

Reports said that 22 South Korean soldiers and one US soldier in South Korea had tested positive for the virus.

As about 28,500 US troops are stationed in South Korea, for the sake of their safety, a "command post training" exercise usually held by the two militaries' Combined Forces Command will be postponed "until further notice", the combined command said in a statement on Thursday.

Experts say the postponement of the drills was inevitable because the potential spread of the virus in military barracks could significantly weaken defense capabilities.

Meanwhile, in the United States, the Centers for Disease Control and Prevention confirmed on Wednesday that a person in northern California has contracted the virus without traveling outside the US or coming in contact with another patient known to have the infection.

US health authorities, managing 60 cases so far — mostly people repatriated from the Diamond Princess cruise ship in Japan — said the US and the world should be prepared for a major outbreak in the upcoming weeks.

Praising China's strict quarantine and containment measures, Bruce Aylward, head of the WHO-China Joint Mission on COVID-19's foreign expert panel, also warned that other nations are "simply not ready" to contain the outbreak.

But US President Donald Trump said on Wednesday that the risk of a major outbreak in the US still remained "very low".

"I don't think it's inevitable," Trump said of the spread of the virus, in apparent contradiction of officials from the CDC, who said a day earlier that the spread of the disease in the US is inevitable.

The president said he recently had a "long talk" with Chinese President Xi Jinping.

"He is working so hard on this problem," Trump said about China's efforts to contain the infection.

Trump also said China has "a significant group of very talented people that are working", and the US is giving them "certain advice", with US experts being part of the WHO team visiting China.

As the virus spreads globally, the impact on large gatherings has increased. The US State Department raised its travel advisory caution level for South Korea to the second-highest level, now urging US citizens to reconsider traveling there. Some media sources also reported that the International Monetary Fund was considering whether to make its April meeting in Washington virtual.

As of Thursday, cases of the virus have appeared in 13 new countries — Algeria, Austria, Brazil, Croatia, Denmark, Estonia, Georgia, Greece, North Macedonia, Norway, Pakistan, Romania and Switzerland — bringing the number of countries hit to more than 46.

Iran has announced over 20 deaths and more than 250 infections, including Mojtaba Zolnour, chairman of the Iranian parliament's national security and foreign affairs committee; Masoumeh Ebtekar, vice-president for women and family affairs; and Iraj Harirchi, deputy health minister. Iranian authorities announced domestic travel restrictions for people with confirmed or suspected cases of the novel coronavirus.

Hadi Khosroshahi, a former Iranian ambassador to the Vatican, died at age 81 on Thursday after he contracted the disease.

A 61-year-old man in Sao Paulo, Brazil, who had recently visited Italy, is Latin America's first confirmed patient.

In addition, Italians or people who recently visited Italy have tested positive in Algeria, Austria, Croatia, North Macedonia, Greece, Romania, Spain, Sweden and Switzerland. The number of infections in Italy, the hardest hit country in Europe, reached 400 late on Wednesday, with 12 deaths.

Agencies contributed to this story.