The FBI has issued an alert warning of cyber attacks by the Syrian Electronic Army, a hacker group allied to the regime of Bashar al-Assad, after it infiltrated systems including those of the US Marines and major news organisations including the New York Times and Associated Press.

The group was able to breach the US Marines recruitment network on 1 September, posting a message urging personnel to disobey orders related to any impending US military intervention in Syria.

In April, the same group was able to corrupt the Twitter feed of the American AP newswire service, posting erroneous bulletins claiming that President Obama had been injured, and briefly causing panic on stock markets.

The FBI warned that the group has developed a new tactic, which had enabled it to compromise outlets including Twitter and the Huffington Post by hacking third-party networks and redirecting visitors to its own site, which carried pro-regime propaganda.

An advisory document issued by the FBI's Cyber Division, headed "Syrian Electronic Army Targeting of Social Media", stated: "The Syrian Electronic Army (SEA), a pro-regime hacker group that emerged during Syrian anti-government protests in 2011, has been compromising high-profile media outlets in an effort to spread pro-regime propaganda.

"The SEA'S primary capabilities include spear-phishing, web defacements, and hijacking social media accounts to spread propaganda.

"The SEA has been highly effective in compromising multiple high-profile media outlets."

The SEA's assault on the Marines website caused acute embarrassment IN the Pentagon. The group was able to post a message aimed at undermining morale.

It read: "Your officer in charge probably has no qualms about sending you to die against soldiers just like you, fighting a vile common enemy. The Syrian army should be your ally, not your enemy.

"Refuse your orders and concentrate on the real reason every soldier joins their military: to defend their homeland."

The group also posted photographs of US troops holding signs denouncing a US-led intervention in Syria, and posted slogans drawing parallels between the US and al-Qaida, which has factions fighting the Assad regime.

The FSA claimed responsibility for its latest attack on Twitter, posting that it had "hacked and defaced" the Marines website.

The FBI advisory, dated 30 August, before last week's attack on the Marines site, added: "In addition to Syrian hackers, groups or individuals sympathetic to the SEA may also be observed participating in CNO [computer network operations] efforts against US websites and networks."

The group hijacked an online service which sells and maintains web addresses, or domain names, enabling it to gain access to web records of news outlets, redirecting visitors to the SEA's own website, which carried pro-Syria messages.

The move left the New York Times website, mobile and tablet apps disabled for much of the day, while Twitter said some of its photo hosting services were affected.

The memo was the FBI's first public acknowledgement of the activities of the hacker group, and comes as the US administration readies itself for air strikes against the regime.