NATO-led forces are expected to cede the lead role for security in Afghanistan this spring to Afghan soldiers, 12 years after the United States invaded Afghanistan to oust the Taliban government harbouring Osama bin Laden, the al Qaeda leader who masterminded the Sept. 11 attacks on US cities.
"No decision has been made but it is an idea that is being considered," Rasmussen told Reuters in an interview, when asked about the possibility of a June summit.
"A summit in mid-2013 could be a good idea to mark the milestone in Afghanistan, the fact that we will have handed over lead responsibility to the Afghans all over Afghanistan," he said.
NATO diplomats say there is talk of organising a one-day summit in Brussels, possibly around the time that US President Barack Obama travels to Northern Ireland for the G8 summit in June, but say no decision has been taken.
Obama announced last week that 34,000 troops, about half the US force in Afghanistan, would withdraw by early 2014, bringing the United States close to wrapping up the costly, unpopular war.
The international force plans to hand over full responsibility for security to the Afghans by the end of 2014, with most international combat forces being withdrawn, leaving a smaller mission to train Afghan soldiers.
Rasmussen said a mid-year summit could also be an opportunity for NATO allies to discuss military capabilities and defence investment, hit in many countries by budget cuts, and to take decisions on the NATO-led training mission planned for Afghanistan after 2014.
Former US President George W. Bush, who ordered the US military intervention in Afghanistan in 2001, came to Brussels for NATO meetings in June 2001 and again in February 2005. NATO leaders last met in Chicago in May last year.
TROOP NUMBERS
Rasmussen, speaking on the eve of a NATO defence ministers' meeting, also said Afghan security force numbers could be kept at a higher level for longer than expected.
Under current plans, the United States and its NATO allies will help build up the Afghan armed forces to 352,000 personnel, a number they are approaching, but the size of the force - which the allies will continue to fund - will be trimmed to 230,000 after 2015.
US General Lloyd Austin, nominated to lead the US military's Central Command which includes Afghanistan, last week backed keeping Afghan forces at peak strength.
Rasmussen said no final decision had been made as to how long Afghan security forces will remain at the high level.
"It may well be that the higher level will be kept for a longer time than previously expected. That will ... very much depend on the security situation on the ground," he said.
Rasmussen said he regretted that US Marine General John Allen would not now become NATO's next supreme commander after he announced his retirement on Tuesday.
Allen, who led NATO forces in Afghanistan and was caught up but later cleared in the scandal that forced CIA chief David Petraeus to resign, cited his wife's health as the reason for his decision to forego his nomination to the NATO job.
"He could have done a great job as Supreme Allied Commander Europe but ... I also fully respect his decision," Rasmussen said.