• The world needs to ramp up efforts to combat COVID-19 — and countries must not let their guard down even as vaccines are rolled out, said Dr. Soumya Swaminathan of the WHO amid a rise in cases.
• The World Health Organization last week warned that new COVID-19 cases were increasing around the world after six consecutive weeks of declines.
• Julie Morita, executive vice president of the Robert Wood Johnson Foundation, also warned that it's not time to be complacent.

The world needs to ramp up efforts to combat COVID-19 — and countries must not let their guard down, the World Health Organization's chief scientist warned Monday as coronavirus cases around the world surge.

"We're at a very risky period," said Dr. Soumya Swaminathan of the World Health Organization. "We need to double down, this is not the period to let up."

The WHO last week warned that new COVID-19 cases were increasing around the world after six consecutive weeks of declines. More than 2.6 million new cases were reported in the last week of February, up 7% from the week before, according to the health agency.

It added that the Eastern Mediterranean, Southeast Asia, Europe and the Americas each saw increases of between 6% and 14%.

"This is partly due to lockdown fatigue, you know. It's due to people … perhaps becoming more casual because they think vaccines are on the way," Swaminathan told CNBC's "Squawk Box Asia" on Monday. New variants may also play a part, she added.

"We need to … do everything we know that keeps these viruses in control, keep transmission down till we have enough vaccines," she said, warning that health-care systems could become overburdened again.

"Health workers around the world are exhausted, they've been fighting this now for over a year," she added.

Other health experts have also expressed that it's not time to get complacent.

Julie Morita, executive vice president of the Robert Wood Johnson Foundation, said it's important to recognize that infections, hospitalizations and deaths are still high, even if they have fallen from their peaks in the U.S.

"It is still necessary for us to wear our masks, to do the social distancing, to avoid large crowds, while we're getting vaccinated," she told CNBC's "Street Signs Asia" on Monday.

"Though the vaccines are ramping up for us in the nation, we cannot let our guard down," she said. "It's way too early to be relaxing."