MANILA - President Rodrigo Duterte's threat to terminate the Philippines’ Visiting Forces Agreement (VFA) with the United States over Washington’s cancellation of Sen. Ronald “Bato” Dela Rosa’s visa is "difficult to understand," Sen. Panfilo Lacson said Thursday.

"Visa is a conditional authorization given by a country to visiting foreigners which may be granted, cancelled, and even denied. The VFA is a bilateral agreement between the Philippines and the US that went through some back and forth discussions prior to Senate ratification but not without intense plenary deliberation," Lacson told reporters in a text message.

"It is therefore difficult to understand the connection between the two."

Asked when how Duterte's threat will affect the relations of the two countries, Lacson said: "It will depend on how the US will react to the President's threat which would say is unfortunate and unnecessary."

Duterte earlier Thursday gave US a month to reverse the visa cancellation of Dela Rosa, who spearheaded his bloody war on drugs during his stint as Philippine National Police chief.

“I am warning you, kapag hindi ninyo ginawa ang correction diyan (if you don’t issue a correction), I will terminate the Visiting Forces Agreement,” the President said in a speech in San Isidro, Leyte.

Meanwhile, Senate President Vicente "Tito" Sotto III said in a statement that the President's threat "can make the US Senate rethink their wrong perception about our justice system."

"He is the President, the buck stops with him."

Dela Rosa on Wednesday confirmed the US revoked his 10-year multiple entry visa but noted that Washington gave no explanation as to when and why his visa was cancelled.

He, however, said the cancellation might be linked to his supposed involvement in the extrajudicial killings (EJK) allegedly perpetrated by police under Duterte’s intensified campaign against narcotics.