Image copyright EPA Image caption President Rodrigo Duterte (right) spoke with Donald Trump on Saturday

Philippines leader Rodrigo Duterte says he may be too "tied up" to accept President Donald Trump's invitation to the White House.

President Duterte said he "cannot make any definite promise" to travel to the US because of a busy schedule with trips planned to Russia and Israel.

Mr Trump extended the offer to Mr Duterte during a "very friendly" call the two leaders had on Saturday.

The Philippines leader has suggested he was open to better US relations.

"I am tied up," Mr Duterte told reporters on Monday when asked about Mr Trump's invitation, even though no firm date has yet been set for such a visit.

"I cannot make any definite promise. I am supposed to go to Russia, I am supposed to go to Israel."

Image copyright Reuters Image caption The White House said the call was "very friendly"

He told reporters that his differences with the US had mainly been with former President Barack Obama, whom he once referred to as a "son of a whore".

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

"Things have changed. There's a new leadership."

During Saturday's phone call the two leaders discussed Mr Duterte's controversial war on drugs, which has left more than 7,000 people dead in under a year.

The North Korea crisis was also raised.

Media playback is unsupported on your device Media caption The Philippines president has called Mr Hussein 'an idiot'

According to the White House, Mr Trump "enjoyed the conversation".

The visit would be a chance "to discuss the importance of the United States-Philippines alliance, which is now heading in a very positive direction", a statement from the White House said.

A meeting between Mr Duterte and Mr Obama was called off last year after he insulted the US president.

Mr Duterte has faced international condemnation for his brutal crackdown on drugs in the Philippines.

Thousands of suspected drug dealers have been killed by police or vigilante groups, and tens of thousands more have been rounded up and arrested.

Mr Duterte - who, like Mr Trump, was elected last year - has also said he would be happy to kill the country's millions of drug addicts.

US Senator Ben Cardin, a Democrat from Maryland and ranking member of the Senate Foreign Relations Committee, called on Mr Trump to rescind his invitation.

"I am deeply disturbed by President Trump's cavalier invitation to President Duterte to visit the White House," he said in a statement.

"Ignoring human rights will not advance US interests in the Philippines or any place else."