By Adrian Croft and Sabine Siebold
BRUSSELS (Reuters) - NATO will decide how quickly to scale back its mission to train Afghan security forces after judging how well they perform in this summer's "fighting season", the alliance's chief said on Thursday.
Afghan government forces, now with little foreign military support, have suffered heavy casualties this year battling the Taliban, who have increased their attacks in the north as well as their traditional southern and eastern strongholds.
NATO pulled out most of its combat troops from Afghanistan last year, leaving around 12,000 personnel to train local soldiers and police in a mission expected to end at the end of 2016.
Before that, NATO trainers are expected to pull back from four regional training hubs and concentrate on the capital Kabul.
The original plan was for the four hubs to be closed by the end of this year, but diplomats say some could now remain open longer.
After talks between defense ministers from NATO and Afghanistan, NATO Secretary-General Jens Stoltenberg said no final decision had been taken on when either phase of the training mission would end.
"We are going to ... assess the situation after the end of the fighting season this year," he said, referring to the period between April and the first winter snows.
By then, NATO would have more information about the security challenges Afghanistan faced and the capabilities of its forces, he said.
Afghanistan has urged the United States, which provides the bulk of foreign soldiers, to slow their withdrawal. U.S. President Barack Obama said in March he would maintain a force of 9,800 through the end of 2015 while sticking to a 2017 exit plan.
Acting Afghan Defence Minister Masoom Stanikzai urged flexibility on troop levels.
"I think everybody realized that we should not only stick with the deadlines, we have to look to the realities on the ground," he told a news conference in Brussels.
German Defence Minister Ursula von der Leyen said it was "important that we judge the situation cautiously and don't leave the country hastily".
After the training mission ends, NATO plans to keep a smaller, civilian-led mission in Afghanistan to advise and instruct security bodies.
Stoltenberg said work was going on to help Afghanistan develop its air force, now that U.S. forces no longer provided air support.
U.S. Defense Secretary Ash Carter said the small numbers of Islamic State fighters in Afghanistan were mostly not new arrivals but fighters who had changed allegiance.