After President Donald Trump shocked human rights observers and members of his own administration by inviting the Philippine president to the White House, Rodrigo Duterte said he might be too busy to visit.

Trump caught both the State Department and National Security Council by surprise Saturday by extending the invitation to Duterte, who has been accused of promoting vigilante violence against suspected drug dealers and users.

But Duterte said Monday that he was unable to commit to the White House visit, reported Agence France-Presse.

"I am tied up," Duterte said. "I cannot make any definite promise. I am supposed to go to Russia, I am supposed to go to Israel."

No firm date had been set for the visit to Washington, D.C.

Duterte said relations between the longtime allies had improved since Trump had taken over for former President Barack Obama, who he called a "son of a whore" after the Democratic president criticized the Philippine president's violent drug war.

“It was not a distancing [of relations] but it was rather a rift between me and the State Department and Mr. Obama, who spoke openly against me," he said.

Duterte, who said Trump had also invited him to visit New York or Washington during a Dec. 2 phone call, has also said he's open to joint military exercises between the Philippines and China.

A $150-million, Trump-branded residential development is set to open soon in Manila.