The Philippines has banned two U.S. lawmakers from visiting and will introduce tighter entry restrictions for U.S. citizens should Washington enforce sanctions over the detention of a top government critic, the president's spokesperson said on Friday.

President Rodrigo Duterte will impose visa requirements on U.S. nationals should any Philippine officials involved in the incarceration of Senator Leila de Lima be denied entry to the United States, as sought by U.S. senators Richard Durbin and Patrick Leahy. 

Duterte is also expected to impose restrictions on U.S. citizens entering the Philippines, requiring they get visas to enter for short-term visits.

The country is preparing to retaliate because of possible sanctions enforced by the U.S. over the detention of a government critic.

The Philippines grants visa-free entry for up to 30 days to Americans, 792,000 of whom visited in the first nine months of 2019, nearly 13 percent of foreign arrivals.

The U.S. embassy in Manila and the State Department did not immediately respond to requests for comment, but Leahy's spokesperson David Carle called the charges against de Lima politically motivated.

Senator Leila de Lima was charged with drug offenses in 2017 after leading an investigation into mass shootings under Duterte's anti-drug campaign. She has been in jail for the past two years.

The U.S. Congress recently approved a 2020 budget containing a provision introduced by Senators Richard Durbin and Patrick Leahy against anyone involved in de Lima's arrest.

"We will not sit idly if they continue to interfere with our processes as a sovereign state," said the Philippine presidential spokesperson Salvador Panelo, according to Reuters.

De Lima is a prominent critic of the Duterte administration, calling for an international investigation against the country's war on drugs.

Duterte makes no secret of his disdain for the United States and what he considers its hypocrisy and interference.

(With input from Reuters)