The World Health Organization declared COVID-19 a global pandemic on Wednesday as the new coronavirus, which was unknown to world health officials just three months ago, has rapidly spread to more than 121,000 people from Asia to the Middle East, Europe and the United States. "In the past two weeks the number of cases outside China has increased thirteenfold and the number of affected countries has tripled," WHO Director-General Dr. Tedros Adhanom Ghebreyesus said at a press conference at the organization's headquarters in Geneva. "In the days and weeks ahead, we expect to see the number of cases, the number of deaths and the number of affected countries to climb even higher." Tedros said several countries have been able to suppress and control the outbreak, but he scolded other world leaders for failing to act quickly enough or drastically enough to contain the spread. "We're deeply concerned both by the alarming levels of spread and severity, and by the alarming levels of inaction," he said, just before declaring the pandemic. "We have rung the alarm bell loud and clear." Cases in China and South Korea have significantly declined, he said, adding that 81 countries don't have any confirmed cases and 57 countries have 10 or fewer cases.

'Lack of resolve'

"We can not say this loudly enough or clearly enough or often enough: All countries can still change the course of this pandemic," he said. "Some countries are struggling with a lack of capacity. Some countries are struggling with a lack of resources. Some countries are struggling with a lack of resolve." Declaring a pandemic is charged with major political and economic ramifications, global health experts say. It can further rattle already fragile world markets and lead to more stringent travel and trade restrictions. WHO officials had been reluctant to declare a global pandemic, which is generally defined as an illness that spreads far and wide throughout the world.

WHO officials needed to "make it clear" that the world was in the midst of a pandemic, said Lawrence Gostin, a professor and faculty director of the O'Neill Institute for National and Global Health Law at Georgetown University. It "is clear" the new coronavirus has been a pandemic and WHO was "behind the curve," Gostin told CNBC on Tuesday.

Changes by the hour

The number of cases and deaths changes by the hour, topping 121,564 with at least 4,373 deaths across the world as of Wednesday morning, according to data compiled by Johns Hopkins University. Outside China, 32,778 cases across at least 109 countries had been confirmed as of 3 a.m. ET Tuesday — up from 282 cases in four countries on Jan. 21, according to the most recent data confirmed by WHO, which tallies the official count. While the virus is slowing in China where it originated in December, it's picking up pace across other parts of the world. Italy has the most cases outside China with roughly 10,149 infections, followed closely by Iran with 9,000 infections and South Korea with 7,775, according to Johns Hopkins University data. In the U.S., cases erupted over the last week to more than 1,050 spread across at least 36 states, according to Hopkins.

H1N1

The last time WHO declared a pandemic was during the 2009 H1N1 swine flu outbreak. Tedros said COVID-19 is the first time a coronavirus has caused a pandemic. The 2002-2003 outbreak of SARS, which is also a coronavirus, was contained enough to avoid that classification. Dr. Mike Ryan, executive director of the WHO's health emergencies program, said health officials take the characterization "very seriously," adding "we understand the implication of the word." "The fact is right now in countries, we have front-line health workers who need our help," Ryan said. "We have hospitals who need our support. We have people who need our care and we need to focus on getting our front-line health workers the equipment, supplies and the training they need to do a good job." "All countries need to reveal their strategies right now," he added.

'You know who you are'

When asked which countries aren't doing enough to combat the virus, Ryan said he wouldn't call out individual countries by name, but added: "you know who you are." Some countries are still using a stringent testing criteria, requiring people to show full symptoms, be over a certain age or somehow linked to travel to China, he said. Some countries haven't been able to stop the virus from spreading within their health-care system or have given up on tracing cases back to their original source, he said. "Some countries have not been communicating well with their populations and creating some confusions in the minds of the populations and risk communication," he said, adding that "trust between governments and their citizens really does need to come to the center."

'Wake up. Get ready'

Epidemics stress every component of a nation, he said. "They stress governance, they stress trust between government and the citizen, they stress the hospital system, they stress public health systems, they stress the economic systems," he said. "In many cases what we're witnessing across society is a lack of resilience." The organization raised its risk assessment level on the virus to its highest level of alert last month. "This is a reality check for every government on the planet: Wake up. Get ready. This virus may be on its way and you need to be ready. You have a duty to your citizens, you have a duty to the world to be ready," Ryan said on Feb. 28. Read CNBC's live updates to see the latest news on the COVID-19 outbreak.