SYDNEY (Reuters) - Australia will seek to strip citizenship from dual nationals who fight with militants overseas or who carry out domestic attacks, Prime Minister Tony Abbott said on Tuesday, the latest in a series of tough new policies aimed at combating the threat from Islamist radicalism.

Australia is on high alert for attacks by radicalised Muslims or by home-grown militants returning from fighting in the Middle East, having raised its threat level to high and undertaken a series of high-profile raids in major cities.

Mr Abbott told reporters that some 100 Australians were fighting in Iraq and Syria backed by about 150 Australia-based "facilitators". Attorney-General George Brandis said that 40 per cent to 50 per cent of those fighting overseas are believed to have dual nationality.

Security analysts have estimated that thousands of foreign fighters are in Iraq and Syria, drawn by the rise of the Islamic State in Iraq and Syria (ISIS) and travelling from scores of countries around the world. "Today I announce that... we will be legislating within a few weeks to strip dual citizens involved in terrorism of their Australian citizenship," Mr Abbott told reporters.

Mr Abbott this week appointed a new counter-terrorism coordinator as part of the new security laws, which are aimed at stopping what he called the "most serious national security challenge that we will face in our lifetimes".

Under tough new security powers won by his conservative government in October, Australian citizens can face up to a decade in prison for overseas travel to areas declared off limits.

Mr Abbott last week ruled out an amnesty for Australian citizens seeking to quit foreign militant groups and return home in the wake of media reports that his government was negotiating with potential defectors.

The new legislation, which is expected to be unveiled within weeks, will update the country's Citizenship Act to target those foreign fighters, as well "lone wolves" in Australia who have been inspired by them to carry out attacks domestically, Mr Abbott said.

The decision to remove someone's citizenship will be subject to judicial review, he said, and will not apply to Australian citizens who do not hold a second passport.

"The new powers will apply to dual citizens who fight with or support groups such as ISIL, or Daesh, as well as so-called 'lone wolves', whether in Australia or on foreign soil," Mr Abbott said in a news release, referring to ISIS.