Philippines President Rodrigo Duterte may reject US President Donald Trump's invitation to the White House, saying he may be too "tied up" to attend.

The controversial figure who has led a bloody drug war fell out of favour with former President Barack Obama, but has appeared to strike up a closer relationship with Trump since he took office.

The pair reportedly held a phone call on Saturday (29 April) where the issue of North Korea was discussed, but where Trump also caused outrage by inviting the leader for a formal visit.

However, speaking to reporters in Davao City on Monday, Duterte said he is unsure if he can attend.

"I'm tied up," he said, reported the Philippines Star. "I cannot make any definite promise."

Duterte referred to previously scheduled trips as a reason for his potential denial, with upcoming trips planned for Cambodia to attend the World Economic Forum and to China for the One Belt, One Road Summit.

He is also due to meet with Russian President Vladimir Putin at the end of May with a later meeting due to be finalised with Israeli Prime Minister Benjamin Netanyahu.

Duterte won domestic approval for his tough-talking style towards the US, but said his conflict was with Obama and not the country as a whole. He previously raised eyebrows when he referred to the US president as a "son of a whore" and told him to "go to hell".

"Things have changed, the new leadership wants to make friends," Duterte said. "[Trump] wants to make friends and he says that we are friends. So, why do you have to pick a fight?"