A conservative advocacy group in the US is up in arms over President Barack Obama's Easter message in which he accused Christians of being "less than loving". 

The American Center for Law and Justice is demanding an apology from the US president over the comments he made in an Easter prayer breakfast address.

"On Easter, I do reflect on the fact that as a Christian, I am supposed to love. And I have to say that sometimes when I listen to less than loving expressions by Christians, I get concerned. But that's a topic for another day," the president said, to laughter and applause from the audience.

The ACLJ is not laughing, however, and neither are the more than 34,000 people who have signed its open letter calling upon Obama to stop his "insults to Christianity".

The letter calls his Easter comments "offensive and uncalled for", and the group is particularly upset that his latest remarks come just months after he suggested Christians get off their "high horse" about Christian persecution during his address at the National Prayer Breakfast earlier this year. 

"We're fighting back – defending Christians – demanding President Obama apologize," the ACLJ said. 

It continued: "Mr. President, Christians aren't 'extremists.' We aren't unloving. We're not on a "high horse" about persecution. Every day Christians die for their faith. They sacrifice their lives for others. Your comments were wrong and hurtful. Stop maligning the Christian faith." 

The president's remarks came just days after the horrific massacre of Christian students at Garissa University College, in Kenya, in which four gunmen murdered 148 people before being killed by Kenyan security forces.

The ACLJ wasn't the only conservative voice blasting the president for his comments. Writing for Fox Nation, radio host Todd Starnes took Obama to task for neglecting to criticize radical Islam. 

"He never bashes Muslims on Muslim holidays, but he seems to take great pleasure in dumping on Christians during our holidays," he said.