Afghanistan ? U.S. and Afghan troops killed at least 40 suspected rebels in an offensive targeting militants who ambushed Navy SEAL commandos and shot down a special-forces helicopter – the deadliest attacks on American forces in Afghanistan, U.S. officials said Monday.

The military declared the weeklong operation in lawless Kunar province near Pakistan a success, following the spate of insurgent attacks that already has made 2005 the bloodiest year for American forces in the country since the Taliban’s ouster.

This year alone, 66 American service members have been killed – more than a third of the 187 who have died in and around Afghanistan since 2001. Four were slain Sunday when a roadside bomb hit a Humvee.

The number of U.S. casualties is a fraction of those suffered in Iraq, yet the barrage of near-daily ambushes, bombings and execution-style killings here has raised fears that almost four years of nation-building is under threat.