One of Germany's biggest terror trials in decades will culminate Thursday in verdicts against self-confessed Islamic militants -- including two German converts -- for a plot to mount attacks against Americans.

The four defendants have each admitted in a high-security courtroom in Duesseldorf, western Germany to a thwarted plan to kill US soldiers, diplomats and civilians in several German cities.

Prosecutors said the planned "massacre" was also intended to punish Germany for its military presence in Afghanistan.

"The plot still sends chills down one's spine," federal prosecutor Volker Brinkmann told the court in closing arguments. "They appointed themselves masters over life and death."

The so-called Sauerland cell, named after the region where the militants were captured in September 2007, has admitted to belonging to a "terrorist organisation", plotting murder and conspiring for an explosives attack.

Prosecutors are seeking jail terms of between five and a half years and 13 years -- less than the maximum 15 years -- due to the defendants' cooperation. Defence attorneys are calling for much lighter judgements.

Before their confessions, the trial had been expected to be one of the lengthiest and costliest in Germany since left-wing urban guerrillas from the Red Army Faction faced court in the 1970s.

Initially due to run two years or more, the trial lasted less than 11 months.

After the biggest surveillance operation in German postwar history, police using US and German intelligence caught three of the suspects red-handed, mixing chemicals to make some 410 kilogrammes (900 pounds) of explosives.

This was 100 times the amount used in the 2005 London bombings that killed more than 50 people, prosecutors said.

The fourth suspect was arrested soon after in Turkey.

Two defendants, Fritz Gelowicz and Daniel Schneider, are German converts to Islam. A third, Atilla Selek, is a German citizen of Turkish origin and the fourth, Adem Yilmaz, is a Turkish national.

The cases of Gelowicz, 30, and Schneider, 24, in particular shocked the country, raising questions how seemingly "normal" Germans could become radicalised by Islamic militant preaching and attend terror training camps.

Gelowicz, Schneider and Selek, 25, have each renounced extremism and described their actions as a "mistake".

"I could have and should have acted differently," Schneider said in a final statement to the court, adding that he hoped to complete a university degree behind bars.

In addition to the other charges, Schneider has also admitted to attempted murder for grabbing the handgun from a police officer while being captured and firing off a shot. No one was wounded.

Yilmaz, 31, also confessed but declined to address the judges during the final hearing.

German authorities say they have gleaned crucial insights during the trial into the Islamic Jihad Union, an Islamic militant group with ties to Al-Qaeda that is believed to have set up training camps in Pakistan.

A federal prosecutor, Rainer Griesbaum, said that thanks to the defendants' confessions "we know how recruitment works, how people are smuggled into the Afghan-Pakistani border region and how the training takes place."

Some 600 officers had been tracking the group's movements around the clock for months when elite commandos swooped on the suspects, holed up with the explosive chemicals and paraphernalia in a holiday cabin.

Authorities said they captured the men just in the nick of time, as they were planning attacks before October 12, 2007, when parliament was to vote to extend German participation in the NATO force in Afghanistan.