The voice, said to be that of Abu Omar al-Baghdadi, leader of the Islamic State of Iraq, was in an audiotape posted on an Islamic Web site routinely used by militant Islamic groups. The authenticity of the tape could not immediately be verified.

The rockets, called al-Quds-1, or Jerusalem-1, "have moved into the phase of military production with an advanced degree of range and accuracy," he said.

The claim that the group was making rockets would be virtually impossible to verify and al-Baghdadi did not elaborate further on the nature of the new weapon.

An alliance of Sunni groups headed by al-Qaeda in Iraq said it has executed 20 Iraqi policemen it kidnapped last week in the north of the country, in an Internet statement posted yesterday.

"After the period expired given by the Islamic State of Iraq to the infidel government of (Iraqi Prime Minister Nuri al-) Maliki about the 20 personnel of the interior and defence ministries, the sharia (Islamic) court has decided to implement the rule of God on them," the Islamic State of Iraq said.

The arms manufacturing capabilities of insurgent groups fighting US and Iraqi forces since 2003 are believed to be very limited, with them relying almost entirely on weapons looted from Saddam Hussein's massive depots in the lawless days and weeks that followed the collapse of his regime.

Weapons are also believed to be smuggled from across the Syrian and Iranian borders.

Hundreds of thousands of Iraqis had been employed in Saddam's huge arms industry, making artillery shells, rifles, land mines, mortars and missiles. The military factories have been abandoned or looted but some of the workers are thought to have joined the insurgency or offered their expertise in the fight against US forces and their Iraqi allies.

Insurgent groups in Iraq have been using a range of Soviet-era rockets like Katyusha and shoulder-fired ground-to-air Sam-7 missiles. The ground-to-ground Katyushas have not been used with great accuracy and anything that the insurgents could make locally was not likely to be an improvement.

Meanwhile, five more US troops have been killed in a series of attacks in Iraq, the military reported Tuesday, taking the losses to 50 in this month alone.

Two marines were killed on Monday in combat operations in the western restive Sunni province of Anbar, the military said.

Another two soldiers were killed and two wounded when their vehicle was struck by a roadside bomb in Fallujah also in Anbar on Saturday. The casualties were announced on Tuesday.

Another soldier died when hit by an explosively-formed projectile (EFP), a kind of a roadside bomb, in southern Baghdad on Monday, a separate statement from the military said.