KABUL, Afghanistan >>  The U.S. military has halted the training of some Afghan forces while it digs deeper into their background following a surge of attacks by soldiers and police on their international partners, officials said Sunday.

The move only puts about 1,000 Afghan trainees into limbo, a small fraction of the country’s security forces. But it shows how these attacks have the potential to derail the U.S.-Afghan handover of security so essential to the international drawdown strategy.

Officials say that the international coalition ultimately hopes to recheck the backgrounds of the entire 350,000-strong Afghan army and police.

The United States and its allies are pushing to have Afghan forces take over security for the country by the end of 2014. This effort has been imperiled by the spike in insider attacks that have killed 45 international service members this year, most of them Americans. There were at least 12 such attacks in August alone, resulting in 15 deaths.

The attacks are straining an alliance already stretched by a tense relationship with a notoriously corrupt Afghan government and disagreements over NATO tactics that Kabul claims endanger civilians.

Coalition authorities have said about 25 percent of this year’s insider attacks had confirmed or suspected links to the Taliban. The militants have sometimes infiltrated the ranks of the Afghan army and police and in other cases are believed to have coerced or otherwise persuaded legitimate members to turn on their coalition partners.

NATO is currently training thousands of Afghans. The 1,000 put into limbo by the training freeze are part of a 16,000-strong unit dubbed the Afghan Local Police. They are actually much more of a government-backed militia, technically under the authority of the national police but operating independently.

They are the only force that the U.S. alone is in charge of training.