Facebook has apologised for accidentally declaring the Philippines was in a state of war, in what was meant to be an innocuous flag tribute for Independence Day.

The social media giant greeted Filipinos a ‘Happy Independence Day’ on Sunday with a post showing a Philippine flag, but it was upside-down with red rather than blue at the top. In the Philippines this officially means the nation is at war.

Facebook, which routinely comes out with greetings to mark holidays around the world, apologised for the blunder.

‘This was unintentional, and we're sorry. We care deeply about the community in the Philippines and in an attempt to connect people on Independence Day, we made a mistake,’ Facebook said in a statement sent to AFP on Tuesday.

The blunder was greeted with a mix of anger and humour in the social media-obsessed country.

‘Dear @facebook: It's not a happy Independence Day if our flag is like this. Like seriously,’ said Twitter user @econcepcion.

But activist Jonas Bagas jokingly linked the gaffe to president-elect Rodrigo Duterte's controversial planned law-and-order crackdown that envisages security forces killing thousands of criminals.

‘We will soon have shoot-to-kill orders and bounties to eradicate the scums that plague our timelines and our streets... Happy Independence Day, Philippines! What a curious state we are currently in,’ Bagas said in a Facebook post.

A nation of over 100 million, the Philippines has 51 million monthly active Facebook users, according to Facebook.

The social network is not the first to cause a stir over an inverted Philippine flag.

In 2010, the United States had to apologise for mixing up the red and blue fields of the Philippine flag during a New York meeting between President Barack Obama and Southeast Asian leaders.

Filipino-American athletes at last year's Southeast Asian Games had a wardrobe malfunction as they wore their vests with the Philippine flags upside-down.

Another sportsman, Seattle Seahawks wide receiver Doug Baldwin, committed a similar blunder in 2013 when he carried an inverted Philippine flag before a game to campaign for victims of Typhoon Haiyan.

Others were not as well-meaning. In May this year, Philippine authorities detained Chinese crew members on the suspicion of poaching after their vessels flew an inverted Philippine flag as they sailed in waters off northern Philippines.