Wikimedia

On the eve of the 13th anniversary of the 9/11 terrorist attacks, Barack Obama laid out plans to extend air strikes against Islamic State militants into Syria for the first time.

The military offensive was unveiled by the US President in an address to the nation on Wednesday night and is being launched to "degrade and ultimately destroy" the ISIS - also called ISIL - militants, whose reign of terror has included beheading two American journalists in recent weeks.

Obama has been reluctant to involve the US in further military action in the Middle East but last night's speech was an acknowledgement from the President of the scale of the threat posed by the Islamic State.

US forces have been carrying out airstrikes on militant targets in Iraq for the last month and Obama vowed that militants would "find no safe haven" in Syria, although he promised that the campaign would not include putting American troops on the ground.

"We will conduct a systematic campaign of airstrikes against these terrorists," said Obama. "I will not hesitate to take action against ISIL in Syria, as well as Iraq."

His speech set a sombre tone for the commemorations taking place today to mark the 13th anniversary of the al-Qaeda attacks on the US in 2001.

The terrorists hijacked four planes and crashed them into the Pentagon and World Trade Center complex in New York City. The fourth plane never made it to its likely target of Washington DC but crashed in a field near Shanksville, Pennsylvania, killing everyone on board.

Thirteen years may have passed but the tragedy in which nearly 3,000 people died still feels fresh in the minds of many and commemoration ceremonies will be taking place at the site in Lower Manhattan and across the US.

The Memorial Plaza will be closed to the public until 6pm as the family of those lost on September 11, 2001 - and in the 1993 World Trade Center attack - come together to remember their loved ones. The names of all 2,983 victims will be read out during the ceremony.

1 World Trade Center, one of four new World Trade Center buildings erected on the site, is set to open this fall.