According to Downing Street, she agreed with Biden "on the importance of protecting" peace in Northern Ireland.
In parliament, Truss said she was "determined" to break through the impasse, and favoured a "negotiated settlement" with the EU.
To Zelensky, Truss vowed to maintain the full-throated support for Ukraine against Russia given by Johnson before he was forced out following a series of scandals.
Truss, 47, won an internal ballot of Tory members on Monday, securing 57 percent of the vote, after a gruelling contest against former finance minister Sunak that began in July.
She now faces a tough challenge reuniting the ruling Tories following the leadership battle, but observers noted that she had expelled almost every Sunak supporter from the cabinet.
Ex-soldier Johnny Mercer said he was "disappointed" to be sacked as veterans affairs minister.
His wife Felicity Cornelius-Mercer went further, calling Truss an "imbecile" as she tweeted a picture mocking the new prime minister as a dim-witted character from "The Muppets".
The Times quoted one of her incoming ministers as saying: "I doubt she'll last two years."
Labour has a double-digit lead in the polls but may have to wait two years to test its popularity.
A general election is due by January 2025 at the latest. Truss on Wednesday again ruled out an early election, mindful perhaps that she needs time to win over a sceptical electorate after Johnson's defenestration.
A new poll by Ipsos found just a third of people expect Truss to do a good job as prime minister, while another third say she will do a bad job.
Source: AFP
