The US visa waiver program allows citizens from 38 approved countries, mostly in Europe, to travel to the United States without a visa for stays of 90 days or less.
There have been increasing calls for doing away with the program, however, following recent attacks in Paris, Ottawa and Sydney.
America's Homeland Security Secretary Jeh Johnson insisted, however, that the program is too "valuable" to discard.
He made his remarks during an annual speech at a US think tank on the progress made in safeguarding US domestic security and future challenges ahead.
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Some US lawmakers and other officials have suggested, however, that the visa waiver program leaves America vulnerable.
US Senator Dianne Feinstein, the top Democrat on the Senate Intelligence Committee, noted earlier this month that the perpetrators of the deadly attack on the Charlie Hebdo magazine were citizens of France -- one of the countries on Washington's visa waiver list.
"It's my belief -- and I have said this publicly many times -- that the visa waiver program is the Achilles' heel of America," she told CNN television, because it puts the nation at greater risk of attack by extremists.
Johnson, however, said that the risks posed by the program, which is constantly being improved, have been overstated.
"We continue to tailor and enhance our security through every appropriate method," said Johnson, calling visa waivers "a valuable tool for international commerce and travel."
"It is a program that must continue, but there are ways in which the security of the program can be improved," he said.
"I've asked my folks to take a hard look at whether... there are additional things that we could ask for, whether they're being complied with. That's under review right now," Johnson said.