Senator Leila de Lima (right) waves to members of the media after casting her vote at a polling precinct in the city of Paranaque in suburban Manila on May 13, 2019. (TED ALJIBE / AFP)

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

De Lima was charged with drug offences in early 2017 after she led an investigation into mass killings during Duterte's anti-drugs crackdown

President Rodrigo Duterte will impose a requirement on US 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 US senators Richard Durbin and Patrick Leahy.

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Duterte's move comes after the US 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 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. 

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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 US 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.