Three suspected terrorists have been arrested for allegedly plotting to bomb Frankfurt airport and the US air force base at Ramstein, German officials said on Wednesday.

Monika Harms, the German federal prosecutor, said the three had trained at camps in Pakistan and obtained some 680kg of hydrogen peroxide for making explosives.

“This would have enabled them to make bombs with more explosive power than the ones used in the London and Madrid bombings,” Joerg Ziercke, the head of Germany’s federal crime office, said at a joint news conference with Ms Harms. The Madrid train bombings killed 191 people, and 52 people died in the London attacks.

The three suspects first came to the attention of authorities because they had been observing a US military facility at the end of 2006, officials said.

All three had undergone training at camps in Pakistan run by the Islamic Jihad Union, and had formed a German cell of the group. The Islamic Jihad Union was described as a Sunni Muslim group based in central Asia that was an offshoot of an extremist group called the Islamic Movement of Uzbekistan.

German media reports said around 20 officers from Germany’s elite GSG 9 unit had stormed a holiday flat being used by the three, two of whom were German converts. The third was believed to be from Turkey.

Der Spiegel online said the investigators made the arrests on Tuesday afternoon after the men were observed moving chemicals from one storage place to another. It is believed that the men wanted to experiment with the chemicals and possibly to start building a bomb. They were, however, some way from making a bomb that could be detonated, Der Spiegel said.

News of the arrests came a day after Danish police arrested eight men suspected of plotting a bomb attack and of having links to al Qaida. They also occurred less than a week before the sixth anniversary of the September 11 2001 attacks in the US.

Frankfurt airport is one of Europe’s busiest airports, with 52 million passengers passing through it each year. The Ramstein base, 80 miles south-west of the airport, is one of America’s most important foreign bases and is a hub for US operations in Afghanistan and Iraq. Germany, which has forces in Afghanistan, has been on high alert for attacks. It has been on the lookout for any re-emergence of militant Islamist groups since 2001, when Hamburg was used as a base for planning the September 11 attacks.

In April, the US embassy in Berlin announced a strengthening of security at diplomatic and military facilities in Germany in response to a perceived increased terrorist threat.

Last year, two men of Lebanese origin attempted to detonate crude suitcase bombs on two German trains, according to German authorities. Prosecutors have said the bombs failed to go off because of a technical error.