Washington is pressing Iraq's leaders to end weeks of political deadlock and form a new government as soon as possible, US officials say.
Mr Jaafari is to be Iraq's new prime minister
Both Secretary of State Condoleezza Rice and Vice-President Dick Cheney have spoken to Kurdish and Shia leaders on the issue in recent days, they say.
Iraqi officials say last minute details delayed announcement of a new cabinet.
US officials are concerned by the political vacuum and fear it is feeding sectarian tensions, correspondents say.
Negotiations on forming a new government for Iraq have taken nearly three months, since the landmark polls at the end of January.
Ms Rice reportedly telephoned influential Kurdish leader Massoud Barzani and, along with Mr Cheney, met Iraqi Vice-President Adel Abdul Mahdi in Washington last week.
"Basically, she was getting an assessment of the coalition and encouraging them to proceed with forming a government," a senior US official was quoted by the Reuters news agency as saying.
The US has previously refrained from getting involved in negotiations over the formation of Iraq's government.
When asked about US unease on the issue, deputy state department spokesman Adam Ereli said only: "The United States and the Iraqis want to see a government formed as soon as possible so that we can keep moving toward fulfilment of the political transition in Iraq."
The European Union has also expressed concern. EU External Relations Commissioner Benita Ferrero-Waldner urged "all sides to really form this government".
Politicians close to Prime Minister-designate Ibrahim Jaafari said on Sunday he was ready to announce a cabinet that would not include his interim predecessor Iyad Allawi.
Mr Allawi, a secular Shia who heads the Iraqi List, announced earlier this month he would join the coalition government and was said to be bidding for at least four ministries.
"I don't see how it can be national unity government without our participation," Iraqi List legislator Hussein al-Sadr told reporters on Monday.