A new security partnership agreement between the United States and Afghanistan is a "significant step" that signals a long-term American presence in Afghanistan, US Defense Secretary Leon Panetta said Monday.

But the terms of a future US and NATO military role -- including how many American troops would remain after a scheduled withdrawal of most combat forces in 2014 -- had yet to be worked out, Panetta told reporters aboard his plane en route to Colombia.

He said the agreement finalized on Sunday "represents a significant step in the relationship and makes clear that the US is committed to an enduring presence in Afghanistan" to help the country stand on its own.

Asked about future troop levels, he said: "That really remains to be worked out, obviously, with our NATO allies and the Afghan government, to determine what the nature of and the size of that force beyond 2014 will be."

Discussions on the details of a follow-on force would likely continue after a NATO summit in Chicago on May 20-21, said Panetta, on the first leg of a trip to South America that include stops in Bogota, Rio de Janeiro and Santiago.

The draft agreement was initialed on Sunday by representatives from Kabul and Washington and will now be reviewed by the US and Afghan presidents, the US Congress and the Afghan parliament.

In Washington, a Pentagon spokesman described the agreement as a statement of "intent" but not a "mutual defense treaty."

"It's a clear indication that the United States will not abandon Afghanistan," Captain John Kirby told reporters.

Officials said the agreement will not specify which bases or airfields US forces will have access to or address the legal status of those remaining in Afghanistan after 2014.

Instead, key details will be hammered out in future deals, officials say.

In Iraq, the United States pulled out all its troops, leaving no residual force, after failing to persuade Baghdad to grant its soldiers immunity from prosecution in local courts.

Panetta and top commanders have suggested a post-2014 force would provide training and assistance in the form of air power, intelligence sharing, logistical support and counter-terrorism operations targeting militants, which could mean roughly 15,000 troops.