The world is fast approaching a tipping point in the spread of the coronavirus, according to experts, who warn that the disease is outpacing efforts to contain it, after major outbreaks forced Italy and Iran to introduce stringent internal travel restrictions and South Korea’s president placed the country on red alert.

Some of the countries most affected by the virus are scrambling to halt its progress two days after Tedros Adhanom Ghebreyesus, director general of the World Health Organization (WHO), said the international community needed to act quickly before the narrowing “window of opportunity” closed completely.

With almost 78,000 cases of Covid-19 now confirmed across the globe, experts say the situation will soon reach a critical threshold.

In 11 north Italian towns, 50,000 people have been in lockdown since Friday night, with police patrolling the streets and fines imposed on anyone caught entering or leaving outbreak areas. In Iran, the authorities have ordered the closure of schools, universities and other educational centres in 14 provinces as “a preventative measure”.

On Sunday, South Korea’s president, Moon Jae-in, placed the country on “red alert” after it reported its fifth death and more than 123 new coronavirus cases, taking the total number infected to 556.

And four of the 32 British and Irish Diamond Princess cruise ship passengers flown to the UK on Saturday have tested positive for Covid-19, the Chief Medical Officer for England said on Sunday. It brings the total number of confirmed cases in the UK to 13.

While the number of patients worldwide is increasing, some virus clusters have shown no obvious link to China, leaving experts struggling to determine where they started.

Paul Hunter, professor in medicine at the University of East Anglia and an authority on the new coronavirus infection, echoed Tedros’s warning and said the time for containing the disease was running out.

“The director general of the WHO has recently spoken of a narrowing of the window of opportunity to control the current epidemic,” he said. “The tipping point after which our ability to prevent a global pandemic ends seems a lot closer after the past 24 hours.”

Hunter said that while cases were declining in China, where the outbreak began, the weekend had seen some “extremely concerning developments elsewhere”.

He said the surge in South Korean cases had been unprecedented so far in the epidemic, adding: “The identification of the large cluster of cases in Italy is a big worry for Europe and we can expect there to be quite a few more cases identified in the next few days.”

Hunter also said the situation in Iran could have major implications for the Middle East. “A further problem with the Iranian cases is wider armed conflicts in the region,” he said.

Dr Robin Thompson, junior research fellow in mathematical epidemiology at the University of Oxford, pointed out that case numbers in Italy had doubled between Friday and Saturday. “This is an important stage of the coronavirus outbreak,” he said. “Fast isolation of even mild cases in affected areas is important for preventing substantial person-to-person transmission in Europe. It is critical that public health guidelines are followed.”

Two people have died from the virus in Italy since Friday and more than a hundred cases have now been reported, most of them centred around the small town of Codogno, about 40 miles south-east of Milan.

Those living in the affected areas now face what the prime minister, Giuseppe Conte, said could be weeks of lockdown. The final two days of the Venice carnival were cancelled and Milan fashion week was also affected.

Iran has recorded eight deaths from the virus, the highest toll of any country outside China. The latest three deaths reported on Sunday were among 15 new confirmed cases, bringing the overall number of infections to 43. Four new infections surfaced in the capital, Tehran, seven in the city of Qom, two in Gilan and one each in Markazi and Tonekabon, the health ministry spokesman, Kianoush Jahanpour, said.

Pakistan responded by closing its land border with Iran, while Afghanistan said it was suspending travel to Iran “to prevent the spread of the novel coronavirus and protect the public”. Turkey also closed its borders and said it would halt incoming flights, adding that all motorways and railways at the border would be shut on Sunday afternoon.

Jordan, meanwhile, will not allow entry to citizens of China, Iran and South Korea and other foreigners travelling from those countries in response to the deadly outbreak.

Announcing his decision to raise South Korea’s alert level on Sunday, Moon said the government and the local authorities should not hesitate to take “unprecedented, powerful” measures to contain the viral disease without being limited by “regulations”.

As his country struggled to contain theoutbreak in the city of Daegu, the president described the coming days as “a very important moment”, the news agency Yonhap said. Of the 123 new cases, 75 were related to the Shincheonji church in Daegu, the country’s fourth-largest city, and a neighbouring hospital.

The Korea Centers for Disease Control and Prevention said they had placed more than 9,000 Shincheonji members in self-quarantine and thousands of worshippers had been screened for the virus.