More than 6,000 jihadists have signed up to fight for Isis (now known as the Islamic State) in Syria since US President Barack Obama authorised airstrikes on the group in northern Iraq, activists have said.

The UK-based Syrian Observatory for Human Rights revealed that the new figures included 1,300 foreign fighters.

It is now estimated that the terror group, which has declared a "caliphate" straddling the Iraqi-Syrian border, now boasts over 30,000 fighters following US military action.

The large increase in its membership base comes after the US launched 167 airstrikes against the group in Iraq since 8 August.

A number of rebel Syrian figures have warned that US military action is actually boosting supporting for the radical Islamic faction.

Spokesman for the Islam Army, Abdurrahman Saleh, said: "Some people become convinced Isis is the right group because Isis is fighting the criminal Assad and the unbelievers in the West."

"Many think Assad is protected by the West and Israel," he added.

"Do things differently"

However, Obama has outlined his plan to "do things differently" in Iraq and not put combat boots on the ground in the Middle East.

"As your commander in chief I will not commit you to fighting another ground war in Iraq," he said.

New York City's Police Commissioner Bill Bratton has warned that the group now present a "very real" threat to the city because of their use of social media to recruit "lone wolf" jihadists who threatened America from within.

Online posts in jihadi forums have called for lone wolf attacks on high-profile US targets such as Times Square.

In retaliation for US airstrikes, the group beheaded two US journalists, James Foley and Steven Sotloff, while releasing a high-quality trailer which warns the American government against putting boots on the ground in the terror organisation's heartland.

Two al-Qaeda offshoots, al-Qaeda in the Arabian Peninsula (AQAP) and al-Qaeda in the Islamic Maghreb (AQIM) have released a joint statement which called upon their jihadist "brothers" in Iraq and Syria to "stop killing each other and unite against the American campaign and its evil coalition that threatens us all".

Elsewhere, Australian police conducted the biggest counter-terror operation in its history after a senior Islamic State militant called for "demonstration killings". Fifteen people were arrested and one person charged with conspiracy to plot a terror attack.

The alleged plot was to carry out a beheading on a random member of the Australian public while covering them in an Islamic State flag.