WASHINGTON -- While the coronavirus pandemic has reshaped U.S. government priorities, Americans must remember that the world remains a dangerous place, Defense Secretary Dr. Mark T. Esper said during a Brookings Institution webinar.

Americans need to concentrate on the virus, but other threats and nations may take advantage of COVID-19 to further their interests, the secretary told Brookings senior fellow Michael O'Hanlon in today's virtual conversation.

"We're still seeing all the same bad behavior out there that we saw before," Esper said.

The secretary noted that Russia is probing air defenses in Alaska and over the North Sea and that the Chinese in the South China Sea are "more pushy" of late.

Both Russia and China are confronting COVID-19, but it is impossible to know the truth about the extent of the pandemic in those countries, Esper said. "They are not reporting it as much, but we know that they're concerned about it," he added.

His message to DOD personnel is to remain vigilant. "These are uncertain times. You don't know how states or militaries will act," he said. "So we [have] got to remain vigilant out there on the front lines."

Esper noted that a key tenet of the National Defense Strategy is dynamic force employment. "It's a way by which you maintain a degree of strategic predictability to ensure the readiness of your force, but garner a higher degree of operational unpredictability," he explained.

The military — even as it's countering the coronavirus — is still taking steps to implement the strategy, the secretary said.

Russia and China are the two main threats to the United States and its allies, followed by Iran, North Korea and violent extremism, Esper said.

To counter China, the United States has changed the bomber presence in Guam, has done more freedom-of-navigation floats and flights and simply has made things more unpredictable for the Chinese, he said. U.S. Indo-Pacific Command has "done a good job in terms of maintaining that show of force, that deterrence, that capability and readiness that we need in the … region," Esper said.

Russia and China are the two main threats to the United States and its allies, followed by Iran, North Korea and violent extremism, Esper said.

To counter China, the United States has changed the bomber presence in Guam, has done more freedom-of-navigation floats and flights and simply has made things more unpredictable for the Chinese, he said. U.S. Indo-Pacific Command has "done a good job in terms of maintaining that show of force, that deterrence, that capability and readiness that we need in the … region," Esper said.

Some of the Chinese provocations may be unprofessional conduct by pilots of sea captains, he said, but some bad behavior is "aggressive actions that are outside the norms of the international rules, whether they're claiming territory or space that simply is not theirs."

"We want to make sure that we maintain, again, the laws of the sea, and the international rules that have sustained us all very well for decades now," the secretary said. "And we see the Chinese continue to try and bend those, to change those and then to shape them in their own favor."

The Russians remain a problem in Europe, Libya and Syria, the secretary said. "I would say with regard to NATO, the alliance has held strong," he said. “I've talked to many of my counterparts from Europe about their state of readiness, how we can help them, etc. But over the last few years, I think we've seen NATO readiness increase. I think overall, the trend for NATO readiness has been positive in terms of capacity, capability and the ability to deploy in a timely manner."

Iran has been hit very hard by the coronavirus, and it's had an impact on the economy and on society, Esper said. "As we've been saying, if they pay more attention to their people, divert their funds to helping the population instead of funding malign activities from Africa all the way through the Middle East, … if they focus their attention, resources on their people, it could be a much better place for the Iranians," he said.