CAIRO (Reuters) - Egypt's most dangerous militant group on Friday posted its first video message since pledging allegiance to Islamic State, with footage purporting to show that the group was behind one of the most deadly attacks on Egyptian security forces in years.

The nearly 30-minute video, whose authenticity could not immediately be verified, was posted on the Twitter feed claiming to represent Sinai-based Ansar Bayt al-Maqdis.

The group this week pledged loyalty to the al Qaeda offshoot now facing U.S. air strikes in Syria and Iraq. It later changed its name to Sinai Province on the Twitter feed, suggesting loyalty to the self-declared Muslim caliphate.

The footage was edited in a slick fashion that resembled videos purportedly released by Islamic State, the ultra-hardline Sunni militants that control swathes of Iraq and Syria.

President Abdel Fattah al-Sisi has expressed concern that an Islamist insurgency in the Sinai could destabilize the Arab world's most populous nation and threaten its neighbours. Militants have killed hundreds of soldiers and police since the army ousted Mursi last year after mass protests against him.

Since last month's attacks, Egypt has stepped up air strikes and created a security buffer zone at its border with the Gaza Strip.

The video shows a man threatening supporters of Sisi. Text at the bottom of the video describes him as the suicide bomber who attacked the Karam al-Kawadis military site.

The man says the fighters "will be the swords that cut your heads." The message appears directed at Egypt's armed forces after a video posted on YouTube last month appeared to show militants beheading three Egyptians accused of being informants for Israeli intelligence.

The man says the group will not let Sisi get away with attacks on Muslims, saying the former army general "has exceeded all limits".

The video cuts to footage of a large explosion in the desert. Gunmen are shown entering an area littered with dead men, some in military fatigues. Another image shows fighters climbing atop a tank and raising the black flag of the Islamic State.

A suicide bombing and a subsequent gun attack at Karam al-Kawadis in northern Sinai on Oct. 24 killed more than 30 security personnel and prompted the government to declare a three-month state of emergency in the area.

At the end of the video, a man in a black robe sits next to a haul of heavy weapons and ammunition that he identifies as spoils of war stolen from the armed forces. He says the war against the state has just begun.

(Reporting by Ahmed Tolba, Lin Noueihed and Maggie Fick; editing by David Gregorio)