The Saudi Kingdom will collapse in two weeks in absence of American protection, US President Donald Trump has said, pressuring one of its closest allies over rising oil prices that are putting huge drain on economies like India.

Trump's reference to Saudi Arabia, which has the potential to strain the bilateral relationship, came in the context of the US providing security protection to rich nations and getting nothing in return.

"How about our military deals, where we protect rich nations and we don't get reimbursed? How about that stuff? That's changing, too, folks," he said.

"We protect Saudi Arabia. Would you say they're rich? And I love the king, King Salman. But I said, 'King, we're protecting you. You might not be there for two weeks without us. You have to pay for your military," Trump said amidst applause from the audience.

"You have to pay," Trump said in an apparent reference to US' friends and allies that they need to reimburse for the security cover America is providing them.

This has been a massive drain on American economy, said Trump, without disclosing when did he speak with the Saudi King.

As crude oil prices reach a four-year high, Trump repeatedly has demanded OPEC and Saudi Arabia, the world's biggest oil exporter, push prices down.

"Japan is going to also contribute. Japan - we protect Japan. They pay us a small percentage. We protect South Korea. They pay us...And by the way, we're doing great on North Korea, but South Korea. They got to reimburse us," Trump said.

On Wednesday, US Secretary of State Mike Pompeo spoke with Saudi Crown Prince Mohammed bin Salman.

"The Secretary thanked the Crown Prince for his continued, strong partnership," State Department Spokesperson Heather Nauert said.

Pompeo and the Crown Prince discussed a broad range of regional and bilateral issues, including Yemen and countering the Iranian regime's malign activities in the region, she said.

The two leaders also discussed areas for expanding US-Saudi collaboration, she added.