The latest intervention by SAS commandos during a rebel attack in Afghanistan will elevate the elite force's pull-out in March higher on John Key's Washington agenda this week.

But the Prime Minister has revealed the SAS wants its mission extended yet again.

Mr Key confirmed the involvement of the SAS in another incident, hours before he left for the United States last night.

Insurgents stormed the Kabul home of Jan Mohammad Khan, an adviser to President Hamid Karzai, on Sunday. Mr Khan and lawmaker Hasham Atanwal were killed in the attack, the BBC reported.

The SAS unit of 38 are mentoring the Crisis Response Unit of the Afghan police.

But in several high-profile attacks, the SAS fighters have been forced to drop the mentoring role and get involved in the action - including a terrorist attack last month on the Kabul Intercontinental Hotel.

The SAS had been due to end its deployment last March, but after pressure from the soldiers themselves, the Government extended their mission to next March.

Mr Key is being careful with his words, saying it is his "expectation" they will come home then. But that leaves the door open to another extension, and yesterday the PM said a final decision had yet to be made.

And he told the Herald: "There's already pressure coming on from the SAS themselves to want to stay longer."

He said it was "unlikely" the force would stay beyond March but refused to categorically rule it out.

"My view is they need to regroup. They will have had two and a half years there."

He said if US President Barack Obama asked about the SAS withdrawal, he would confirm the intention to withdraw.

"If he asks, that will be the point I am making. Round one is our SAS coming out and round two is the provincial reconstruction force coming out at the end of 2014."

Others like the Canadians were leaving and the British and French were cutting their numbers. "It's quite a delicate time in Afghanistan," Mr Key said.

The PM said that in the latest incident - a three-hour fight between security forces and the insurgents - the SAS began in a mentoring role but later became actively involved.

One Afghan security force member was killed and another was injured.

Mr Key said that as far as he knew, the SAS troops had not suffered any casualties, although two insurgents died.

"We're in the worst of the season in Afghanistan at the moment, the summer fighting season where the number of attacks intensifies, therefore the New Zealanders are becoming more involved."

As well as meeting President Obama early on Saturday (NZT), Mr Key is also scheduled to meet the new United States Secretary of Defence, Leon Panetta.