U.S. Secretary of State Rex Tillerson has underscored Washington's intent to strengthen ties with Australia and South Korea, the State Department said Tuesday, a move aimed at reassuring allies unnerved by the campaign rhetoric of new President Donald Trump.

In separate calls with counterparts from the longtime allies, they agreed to work closely to tackle threats from North Korea's nuclear ambitions and increased tensions in the East and South China seas, State Department spokesman Mark Toner said.

"Secretary Tillerson reiterated the administration's intent to strengthen our military alliances, our economic partnerships and our diplomatic cooperation," he said in a statement.