U.S. President Donald Trump on Sunday stepped up outreach to allies in Asia to secure their cooperation to pressure North Korea over its nuclear and missile programs.

Trump spoke to the prime ministers of Thailand and Singapore in separate phone calls about the North Korean threat and invited both of them to Washington, U.S. officials said.

"They discussed ways to maintain diplomatic and economic pressure on North Korea," one U.S. official said of the calls, speaking on condition of anonymity.