US President Donald Trump called Prime Minister Lee Hsien Loong on Sunday (April 30) evening for their second conversation since Mr Trump won the presidency, and invited him to the White House to "further strengthen ties".

During his May Day Rally speech, PM Lee said he has accepted the invitation and hopes to make the trip some time this year.

"I've just had a good phone call with President Trump last night and he invited me to go to Washington," he said during the speech to Labour Movement leaders on Monday (May 1). "I've agreed and I hope to do it some time this year."

PM Lee and President Trump "affirmed the deep and longstanding relationship between Singapore and the United States", a statement from Singapore's Ministry of Foreign Affairs said after the Sunday call.

The brief statement added: "They expressed satisfaction that ministers from both sides are already working with each other and looked forward to meeting each other soon."

The White House later issued a statement saying Mr Trump had invited Mr Lee to the White House "to further strengthen ties", noting that the partnership between Singapore and the US "is marked by thriving trade and investment, robust security cooperation, and close collaboration on regional and global challenges".

The two leaders' first call was on Dec 2 last year.

Mr Trump also called Thailand's Prime Minister Prayut Chan-o-cha, the White House said in a separate statement, underscoring "their commitment to the longstanding alliance between the US and Thailand, which actively contributes to peace and stability in the Asia-Pacific region".

"President Trump affirmed the commitment of the US to playing an active and leading role in Asia, in close cooperation with partners and allies like Thailand, and invited Prime Minister Prayut to the White House," said the statement.

The calls to regional leaders aimed to get "everyone in line backing up a plan of action" if the situation in North Korea deteriorated, White House Chief of Staff Reince Priebus told ABC News.

"We need cooperation at some level with as many partners in the area as we can get to make sure that we have our ducks in a row," he noted.

The intention was to get "everyone in line backing up a plan of action" if the situation in North Korea deteriorated, he said.

He added that Mr Trump was in regular contact with Japan's Prime Minister Shinzo Abe and had become "very close" to Chinese President Xi Jinping.

"There is nothing right now facing this country and facing the region that is a bigger threat than what is happening in North Korea" Mr Priebus said, adding that the conversations were prompted by the "potential for nuclear and massive destruction in Asia" and eventually in the US.

Earlier on Saturday on CBS's Face the Nation programme, President Trump appeared to downplay the threat while keeping the world guessing on his strategy. Asked if he would consider military action in response to another nuclear test, he responded: "I don't know. I mean, we'll see."

And when asked the same question during a factory tour before his rally in Harrisburg to mark his 100th day in office, he told reporters: "You'll soon find out."

The missile launched on Saturday blew up over land in North Korean territory. But Mr Trump tweeted "North Korea disrespected the wishes of China & its highly respected President when it launched, though unsuccessfully, a missile today. Bad!"

The calls also came ahead of a May 4 meeting of foreign ministers from Asean in Washington DC with Secretary of State Rex Tillerson - this US administration's first high-level meeting with the 10-member group.

While not once travelling abroad, President Trump has had 16 bilateral meetings with other world leaders, including Mr Abe and Mr Xi. He has spoken subsequently with the Japanese and Chinese leaders on the phone over the crisis on the Korean peninsula.

A new, less ideological and more transactional foreign policy has emerged from the calls and meetings, analysts say.

President Duterte for instance, openly insulted former US President Barack Obama when the latter criticised his crackdown on drugs; with President Trump that dissonance has vanished. Their phone conversation on Saturday - the second since Mr Trump won the election - was "very friendly", the White House said.