US President Donald Trump has caused outrage by boasting that American service members killed in action would be “happy” with the way he was running the country.

“Happy Memorial Day! Those who died for our great country would be very happy and proud at how well our country is doing today. Best economy in decades, lowest unemployment numbers for Blacks and Hispanics EVER (& women in 18 years), rebuilding our Military and so much more. Nice!” he tweeted on America’s Memorial Day, which honours military service men and women killed while serving.

Many, including veterans groups were outraged over Mr Trump’s self-serving tweet on a day set aside to honour and mourn America’s war dead, the New York Post reported.

“This is the most inappropriate #MemorialDay comment that a @PotUS has ever made. Self-promotion on a day to remember the fallen, and wishing those remembering their deceased loved ones a ‘happy’ holiday is appalling. #CadetBoneSpurs,” said veterans group Vote Vets.

The hashtag refers to the nickname given to Mr Trump by politician Tammy Duckworth, a retired Army lieutenant colonel who lost both legs in combat, after Mr Trump said he was given a medical pardon not to serve in the Vietnam War because of “bone spurs” in his heels.

Twitter user Christina Marie, whose father died serving, tweeted at the president:

“My father, who succumbed to his severe PTSD from Vietnam (which you dodged the draft for) would be sickened by your words against his home country, El Salvador. You don’t know sacrifice or dedication to this country.”

“Putting the ‘me’ in Memorial Day,” quipped news anchor Chris Cillizza.

“Memorial Day is to remember the men and women who are the reason we are where we are. Not for taking any credit for yourself,” user Kathy Walker said.

“You’re making this national holiday, a day of remembrance for our fallen service members about YOU? Just when I think you’ve hit bottom, you manage to find a new way to go even lower,” echoed Will N.

Later, during the annual Memorial Day ceremony at Arlington National Ceremony in Virginia, a bugler played taps as the wreath was laid, before the president delivered a more traditional address.

“We are gathered here on the sacred soil of Arlington National Cemetery to honour the lives and deeds of America’s greatest heroes, the men and women who laid down their lives for our freedom,” Mr Trump said.

“Today, we pay tribute to their service, we mourn alongside their families, and we strive to be worthy of their sacrifice. The heroes who rest in these hallowed fields, in cemeteries, battlefields near and far are drawn from the full tapestry of American life, and they were all united then as they are united now forever by their undying love of our great country.”

Meanwhile, other politicians tweeted to honour veterans on Memorial Day.

“Today we honour the Americans who sacrificed everything to secure the blessings of liberty. Family and friends to some, heroes to all — who lived, fought and died for the safety and future of a great and good nation. God bless them and grant them perpetual peace,” wrote ailing Senator John McCain, who Mr Trump once said was not a war hero because he was captured.

“Our men & women in uniform are the heart, the soul, and the spine of this nation. Today we remember those among them who gave their lives for our freedom. As we honour their sacrifice, we must recommit to our sacred obligation to care for & support the families they left behind,” added former vice president Joe Biden.

Trump’s campaign website also offered a 25 per cent discount during its “Memorial Day Sale.”

“You ONLY have a few hours left to get 25% off ALL Official Trump Store purchases before the Memorial Day sale is OVER,” the site reads.