BAGHDAD, Iraq – Three helicopters have been lost in Iraq in the past 10 days, including one over the weekend. While insurgents and militias have plenty of weapons capable of shooting down helicopters, the U.S. has developed tactics to minimize the risk, and many experts doubt the threat to the military’s workhorse has significantly increased.

Since May 2003, the U.S. military has lost 54 helicopters in Iraq, about half of them to hostile fire, according to figures compiled by the Brookings Institution. In addition, an OH-6A helicopter owned by the private security company Blackwater USA crashed last Tuesday in Baghdad in heavy gunfire, killing four civilian contractors.

The private aircraft was among three helicopters which have crashed in Iraq since Jan. 20. They also include a U.S. Army Black Hawk that crashed in Diyala province, killing 12 soldiers, and an unspecified army helicopter that went down Sunday during heavy fighting near Najaf, killing the two crew members.

Both helicopters were believed to have been shot down, although the U.S. military will not yet confirm the cause.However, a U.S. military official in Washington said the helicopter that crashed near Najaf was shot down by small arms fire rather than any sophisticated anti-aircraft weapon.

The recent spate of losses raises questions about whether Sunni insurgents and Shiite militias may have stepped up attacks on helicopters or may have received new supplies of weapons. Sunni insurgents are already known to have SA-7 anti-aircraft weapons in their arsenal as well as rocket-propelled grenades and heavy machine guns.

If the insurgents have new weapons or have decided to step up attacks on aviation, that would be a major problem for U.S. commanders, who rely heavily on helicopters not only in combat but also to move soldiers and supplies around the country.