MANILA (Reuters) - 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 spokesman said on Friday.

President Rodrigo Duterte will impose a requirement on U.S. nationals to get visas 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’s move comes after the U.S. Congress approved a 2020 budget that contains a provision introduced by the senators against anyone involved in holding de Lima, who was charged with drug offences in early 2017 after she led an investigation into mass killings during Duterte’s notorious anti-drugs crackdown.

“We will not sit idly if they continue to interfere with our processes as a sovereign state,” Philippine presidential spokesman Salvador Panelo told a regular news conference.

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% of foreign arrivals, government data showed.

The U.S. embassy in Manila did not immediately respond to a request for comment.

Panelo said travel restrictions over de Lima’s detention were nonsense because she was not wrongfully imprisoned but detained pending trial for crimes.

“The case of Senator de Lima is not one of persecution but of prosecution,” he said.

Duterte makes no secret of his disdain for the United States and what he considers its hypocrisy and interference, though he admits that most Filipinos and his military have high regard for their country’s former colonial ruler.

The United States is the Philippines biggest defense ally and its main source of Western influence. Millions of Filipinos have relatives who are U.S. citizens.

De Lima, a justice minister in a former administration, on Wednesday expressed what she described as overwhelming gratitude to the U.S. Congress for its help.

She has won numerous awards from human rights groups, who consider her a prisoner of conscience.

She has constantly spoken out against Duterte and been calling for an international investigation into his war on drugs, in which thousands of people have been killed.

Police say those killed were drug dealers who resisted arrest, but activists believe many of the killings were murders.