American conservatives have voiced their disappointment after President Joe Biden, a Catholic, issued a National Day of Prayer proclamation this week that did not explicitly mention God.

The proclamation invited people "to give thanks, in accordance with their own faiths and consciences, for our many freedoms and blessings."

It calls the US a nation "of remarkable religious vitality and diversity" and attributes this to "the right of all Americans to pray".

"On this National Day of Prayer, we unite with purpose and resolve, and recommit ourselves to the core freedoms that helped define and guide our Nation from its earliest days," the proclamation says.

"We celebrate our incredible good fortune that, as Americans, we can exercise our convictions freely — no matter our faith or beliefs.

"Let us find in our prayers, however they are delivered, the determination to overcome adversity, rise above our differences, and come together as one Nation to meet this moment in history."

But nowhere does the proclamation actually say 'God', in contrast to those issued by Biden's predecessor Donald Trump and the previous Democratic President Barack Obama. 

Evangelical leader Franklin Graham said he was "deeply saddened" by this omission. 

"That speaks volumes doesn't it? It is hard to believe we have come this far," he said in a Facebook post.

"Omitting God is a dangerous thing. He is who we as individuals and as a nation need to call on for help. He created us and made salvation possible for us through faith in His Son, Jesus Christ.

"Prayers to anyone or anything else are to no avail. Join me in praying to Almighty God on behalf of our nation.

"Your kingdom come, Your will be done, on earth as it is in heaven."

Conservative radio host Todd Starnes, tweeted, "Regarding Biden's godless National Day of Prayer proclamation: The problem with the Democrats is that they don't believe that we are endowed by our Creator with certain unalienable rights. They believe we are endowed by the government."