Ashraf Ghani's government will sign a long-delayed deal on Tuesday to allow some US troops to stay in Afghanistan into next year, US and Afghan officials said.

The BSA (bilateral security agreement) became a symbol of frayed US-Afghan ties when President Hamid Karzai refused to sign the deal last year, infuriating Washington and other international allies.

"The BSA will be signed tomorrow, not by the president but by a senior minister," Daoud Sultanzoy, a senior aide to Ghani, told AFP.

"The signing sends the message that President Ghani fulfills his commitments. He promised it would be signed the day after inauguration, and it will be."

State Department spokeswoman Jen Psaki confirmed that the two countries had agreed to sign the deal on Tuesday.

US Ambassador James Cunningham will be "signing on behalf of the United States," she said.

"This will enable Afghanistan, the United States and the international community to maintain the partnership we've established to enure Afghanistan maintains and extends the gains of the past decade," Psaki told reporters.

Karzai had declined to sign the pact, even though it has popular support in Afghanistan and was approved by a traditional grand council or Loya Jirga.

Ghani was inaugurated on Monday in Afghanistan's first democratic transfer of power as Karzai stepped down.

Under the bilateral security deal, the US deployment in Afghanistan will be scaled back to around 9,800 by the start of 2015.

Those forces will be halved by the end of 2015, before being reduced to a normal embassy presence by the end of 2016.

Along with troops from NATO allies including Germany and Italy, the US-led mission in Afghanistan in early 2015 will be about 12,500-strong.