Trump, in his separate phone calls to Theresa May and Malcolm Turnbull yesterday, ruled out any talks with North Korea over its nuclear weapons programme, it said.
The US president has been making phone calls to other world leaders as well since the North detonated what it calls a hydrogen bomb, the biggest nuclear device it ever tested.
Trump called South Korean and Japanese leaders several times over the week to discuss the crisis on the peninsula.
"President Trump reiterated that now is not the time to talk to North Korea, and made clear that all options remain open to defend the United States and its allies against North Korean aggression," it said.
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Trump and Turnbull condemned North Korea's "belligerent actions" and confirmed that their two countries will intensify joint efforts to denuclearise North Korea.
"President Trump reaffirmed his commitment to defending the homeland, territories, and allies of the United States, using all available diplomatic and military capabilities," the White House said.
"Mattis assured minister Song that the US remains ironclad in its commitment to the defence of the ROK (South Korea). He further emphasised that any threat to the United States, its territories, or its allies will be met with a massive, effective, and overwhelming military response," Pentagon chief spokesman Dana W White said.
"Mattis and Song discussed avenues for improving bilateral, trilateral, and regional defence cooperation in light of the threat posed by North Korea and to maintain peace and stability in Northeast Asia and the broader Asia-Pacific region," the White House said.