BAGHDAD — The American military announced six new deaths yesterday, making 2007 the bloodiest year for American troops in Iraq despite a recent decline in casualties and a sharp drop in roadside bombings that Washington links to Iran.

With nearly two months left in the year, the annual toll is now 853 — three more than the previous worst of 850 in 2004.

But the grim milestone comes as the Pentagon points toward other encouraging signs as well — growing security in Baghdad and other former militant strongholds that could help consolidate the gains against extremists.

A senior Navy officer, meanwhile, announced the planned release of nine Iranian prisoners and was at pains to say that a major cache of Iranian-made weapons and bombs displayed for reporters yesterday appeared to have been shipped into Iraq before Tehran made a vow to stop the flow of armaments.

Defense Secretary Gates said last week that Iran had made such assurances to the Iraqi government. He did not say when the pledge was issued. A decline in Iranian weapons deliveries could be one of several factors for the decrease in both Iraqi and American deaths over the past two months.

“It’s our best judgment that these particular EFPs … in recent large cache finds do not appear to have arrived here in Iraq after those pledges were made,” Rear Admiral Gregory Smith, the director of the Multi-National Force-Iraq’s communications division, told reporters yesterday.

Among the weapons Washington has accused Iran of supplying to Iraqi Shiite militia fighters are EFPs, or explosively formed projectiles. They fire a slug of molten metal capable of penetrating even the most heavily armored military vehicles, and thus are more deadly than other roadside bombs.

The no. 2 American commander in Iraq, Lieutenant General Ray Odierno, said last week that there had been a sharp decline in the number of EFPs found in Iraq over the last three months.