New Zealand could send more troops to Afghanistan but would only do so as part of an overall exit strategy, Prime Minister John Key said this morning.

The United States has formally asked for New Zealand's SAS to be sent to Afghanistan.

The request followed a meeting Foreign Minister Murray McCully had with US Secretary of State Hillary Clinton in Washington earlier this month where he drew the inference she wanted the SAS back.

John Key said the United States had a global "shopping list" to institute a "surge type plan, on the basis that they had in Iraq, to try and get on top of the situation and then ultimately to exit out".

"We could do it. Why would we do it? Well I guess the argument would be if it was part of an exit strategy - I don't want to stay in Afghanistan forever," Mr Key told TV One this morning.

New Zealand already has a large contingent in Afghanistan and would need more details of the US plan before making a further commitment, he said.

He rejected claims the Government would supply the extra troops to keep the US happy.

"We have 140 people there, we can't leave them there in isolation ... The challenge is how do we get out of Afghanistan."

Yesterday, Mr McCully said there was a military and a civilian component to the request. "They've sought specifically special forces, SAS," Mr McCully told TV One's Q&A programme.

Resourcing and coverage were issues to consider.

"We're looking at those issues alongside the SAS deployment and saying if something else happens somewhere else closer to home in our region, what is our capacity to react," he said. "Remember Afghanistan is not our biggest deployment, Timor-Leste is. We've got significant numbers of people in the Solomons, we've seen trouble in Tonga, we've seen trouble in other places."

Mr McCully said he was not including Fiji in that scenario.

If the Government agrees, the elite troops will be on their fourth mission to Afghanistan. They were last there in 2006.

The Government was going to take into account the rollover of the provincial reconstruction team of about 140 Defence Force personnel that was in Afghanistan and would be there until at least September next year.

He would not say when the Government would make a decision but referred to the Government's defence review which is due to be completed by August.

That coincides with elections in Afghanistan, when the country will be vulnerable to unrest.