US President Donald Trump on Thursday said national parks would hike entry fees for foreign tourists to "improve affordability" for Americans, as he launched the country's year-long 250th birthday celebrations.

"For this anniversary, I've just signed an executive order to raise entrance fees for foreign tourists while keeping prices low for Americans," Trump told a cheering crowd at a rally in Iowa.

"The national parks will be about America First," the Republican leader said, after issuing an executive order.

In it, Trump also instructed the interior and state departments to "encourage international tourism to America's national parks."

The order outlined that revenue raised was to be used to improve the infrastructure and "enhance enjoyment" of the country's vast national park system.

It is a rare move by the climate sceptic president to promote the environment and green spaces.

In the executive order, Trump also revoked a 2017 directive by former president Barack Obama on "promoting diversity and inclusion in our national parks," in his latest attack on DEI (Diversity, Equity and Inclusion) initiatives.

Some conservation groups have however voiced concerns about hundreds of National Park Service permanent staff members being laid off since Trump took office in January, ahead of peak tourist season in summer.