"This signifies the successful completion of the third step in Iraq's constitutionally mandated government formation process, following the election of the speaker of the parliament and the election of the president of the republic in July," Xinhua quoted Kerry as saying in a statement.
Masoum on Monday asked Abadi, deputy speaker of the parliament, to form the next government.
Politicians allied to outgoing Prime Minister Nuri al-Maliki Monday rejected Abadi, saying he is not representing them and has no legitimacy.
The US applauds Masoum's fulfilment of his constitutional duties and urges Abadi to form a government that is representative of the Iraqi people and inclusive of Iraq's religious and ethnic identities, Kerry said.
"The prime minister-designate should present the members of his proposed new government to the council of representatives consistent with Iraq's constitutional timeline," the top US diplomat added.
In a telephonic conversation Monday, US Vice President Joe Biden commended Masoum for reaching the milestone of nominating the country's next prime minister.