Clarification: A previous version of this story listed Canada as one of the countries that cautioned travelers about gun violence in the United States. The Canada travel advisory says "incidents of mass shooting occur, but account for a small percentage of homicide deaths in the country. The likelihood of a tourist being a victim of such an incident is low."

WASHINGTON – President Donald Trump threatened undefined retaliation Friday against countries and organizations that issue travel warnings on the United States because of gun violence.

"If they did that, we'd just reciprocate," Trump said during a wide-ranging impromptu gaggle with reporters at the White House, en route to fundraisers in New York.

He added: "We are a very reciprocal nation with me as the head. When somebody does something negative to us in terms of a country, we do it to them."

Amnesty International and a growing list of countries have begun issuing warnings about travel to the United States because of gun violence, including mass shootings over the weekend in El Paso, Texas, and Dayton, Ohio.

Know the risks:Travel advisories by country

Warnings:Amnesty International issues warning about travel to US amid 'high levels of gun violence'

Other countries in recent yearshave cited U.S. gun violence in travel advisories, with varying degrees of intensity. They include Uruguay, Venezuela, Japan, Germany, Ireland, and New Zealand.

In an advisory issued Wednesday, Amnesty International said: "Depending on the traveler's gender identity, race, country of origin, ethnic background, or sexual orientation, they may be at higher risk of being targeted with gun violence, and should plan accordingly."

Trump:Congress discussing 'meaningful' gun background checks, NRA will have input

Trump did not specify any particular country, or what he might do to retaliate.

Trump also said he received another "beautiful letter" from Kim Jong Un, in which the North Korea leader protested recent "war games" – joint military exercises – between the United States and South Korea.

Trump said he sympathized with Kim's complaint, saying he "never liked" the exercises because "I don't like paying for" them. He said he approved the recent round of exercises because he thought they were necessary.

Trump also questioned whether September trade talks would take place between the United States and China, as they are locked in a renewed tariff war.

Trump and the NRA:Trump spoke with NRA CEO Wayne LaPierre during Dayton, El Paso trip, faces opposition on gun background checks

While both sides still want a trade agreement, Trump said, "we'll see whether or not we keep our meeting in September ... We are winning and winning big."

The president said the nation's children should not worry as they head back to school this fall, even amid a spate of mass shootings in recent years.

"They have nothing to fear,' he said. "They have nothing to worry about."