Germany should brace for further attacks given growing numbers of potential Islamist militants, top security officials warned on Tuesday as they vowed to step up efforts to prosecute, convict and deport suspects.

The country is still home to 24,400 Islamists, according to a report from Germany's BfV domestic intelligence agency.

Germany was hit by five Islamist attacks in 2016, including a December attack on a Berlin Christmas market that killed 12 people.

Germany is still home to 24,400 Islamists, according to a report from the country's BfV domestic intelligence agency. Hans-Georg Maassen, president of Germany's BfV domestic intelligence agency, said the country should expect more terror attacks

Meanwhile, German police have increased their presence in and around Hamburg in preparation for the Group of 20 summit in the city on Friday and Saturday

Seven additional attacks failed or were thwarted, Hans-Georg Maassen, president of Germany's BfV domestic intelligence agency, told reporters.

'We must expect further attacks by individuals or terror groups,' Maassen said, citing growing evidence and over 1,000 expressions of concern from the general public about growing risks.

'Islamist terrorism is the biggest challenge facing the BfV and we see it as one of the biggest threats facing the internal security of Germany,' he said.

The agency's annual report for 2016 said there were 24,400 Islamists in Germany, including around 9,700 Salafists, and the number of Salafists had increased to 10,100 this year.

The total also includes some 10,000 members of the Turkish Islamist Milliu Gorus movement, the report showed.

The total number of suspected Islamists marks a drop from the year earlier, but the report said that did not mean the threat had diminished.

German Interior Minister Thomas de Maiziere (center), Hamburg's Interior Senator Andy Grote (center right), Chief Police Officer of the Federal Police Thomas Przybyla (center left) and G20 Head of Operations Police Chief Hartmut Dudde (center second right) pose beside a special unit of the Federal police on Tuesday at the Messehallen convention centre in Hamburg, northern Germany, the venue of the two-day G20 summit

'In fact the opposite is the case,' the report said, citing a shift towards 'a more violence-prone and terrorist spectrum ...'

The report said hundreds of 'jihadists' had entered the country among the over one million migrants who had come into Germany over the past two years.

Altogether, security officials were keeping tabs on some 680 potential Islamist threats, most of whom were influenced by Salafist ideology, Interior Minister Thomas de Maiziere said.

He said Germany had dramatically stepped up its efforts to combat Islamistic militancy, with a record number of arrests, prosecutions and deportations seen over the past year.

Maassen said an estimated 930 people had left Germany to fight with Islamic State in Syria or Iraq, of whom about 20 percent were women.

Police have already seized knives, baseball bats and presumed incendiary devices from various locations in and around Hamburg

An estimated 145 of the total people had since died, he said.

Meanwhile, German police have increased their presence in and around Hamburg in preparation for the Group of 20 summit in the city on Friday and Saturday.

They have already seized knives, baseball bats and presumed incendiary devices from various locations.

Authorities expect about 8,000 violent protesters to converge on Hamburg as Chancellor Angela Merkel hosts the leaders of 20 major advanced and developing economies, Interior Minister Thomas de Maiziere said on Tuesday.

Some 20,000 police will be on duty.

'There is evidence that the acts of violence around the G20 summit that we had expected and feared will take place,' said Ralf Martin Meyer, president of Hamburg's police.

Senior police officer Jan Hieber said police had probably only found a small roportion of the weapons that had been stockpiled for use in the protests.