President Barack Obama has said he would like to further cut the US nuclear arsenal, stating, however, he has concerns about efforts to modernise America's most deadly weapons.

"My preference would be to bring down further our nuclear arsenal," he told reporters at the end of a Nuclear Security Summit he hosted in Washington, RTE news reported from Ireland.

Obama won the Nobel Peace Prize in his first year in office for his commitment to non-proliferation and was responding to a question about updates to the U.S. arsenal

The U.S. president used his final nuclear security summit on April 1 to deliver a stark warning that "madmen" could kill and injure hundreds of thousands of innocent people using only plutonium the size of an apple.

"The danger of a terrorist group obtaining and using a nuclear weapon is one of the greatest threats to global security," said Obama, convening the meeting of more than 50 world leaders in Washington, The Guardian reported.

Obama argued that since the first such summit six years ago, the world has measurably reduced the risk of nuclear terrorism by taking "concrete, tangible steps". Enough material for more than 150 nuclear weapons has been secured or removed, he said.

Absent from the summit is Russia which thought to possess more nuclear weapons than any other country, including the United States. The two former Cold War foes share about 90 percent of the world's nuclear arsenal.

In this case President Vladimir Putin was the spoiler, but Rev. Stephen Sidorak is a U.S. pastor who no matter what continues his decades-long toil for his yearning that nuclear weapons must never be used in anger again.

Last year he carried out a "Pilgrimage of Justice and Peace to Japan on a Matter of Life and Death" in decades of toiling for his yearning that nuclear weapons must never be used in anger again.

When the World Council of Churches Commission of the Churches on International Affairs met in Geneva March 7-11 Sidorak spoke to participants about a pilgrimage he made to Japan 70 years after the atomic bombings.

Sidorak is ecumenical staff officer for the Council of Bishops Office of Christian Unity and Interreligious Relationships of the United Methodist Church in the United States.

He notes that recent actions on the Korean Peninsula underline the fragility around nulcear armaments use.

He traces his own lengthy involvement in efforts to rid the world of nuclear weapons to his time in parish ministry before the pilgrimage on the 70th anniversary of the dropping of nuclear bombs on Japan in the closing days of the Second World War.

MISSIONS TO HIROSHIMA AND NAGASAKI

"A member of a local church I served recounted his duties during the Second World War which included flying the cover squadron escort missions over Hiroshima and Nagasaki," in August 1945 shortly after the bombings.

"He was the pilot of the plane assigned to document the destruction of the two Japanese cities," said Sidorak elaborating on the only two instances of cities having atomic bombs dropped on them.

"The primary reason for our pilgrimage to Japan was to address our contemporary nuclear predicament both in terms of weaponry and energy," explained Sidorak.

The world needs to move forward on this issue, he says.

The visit to Japan involved "visiting the wounds," and visiting "locations of ugly violence and injustices."

At least three locations came to mind as the pilgrims arrived in Japan - Hiroshima, Nagasaki and Fukushima (which was ravaged by a massive earthquake, tsunami and nuclear disaster in March 2011).

Many Christians today believe idolatry to be a thing of their pagan past.

Sidorak says, however, there is new idolatry "as we now worship the power we have to destroy creation much more than we worship the God of creation".

"We bow down and worship weapons of mass destruction and the source of their power, nuclear energy," he notes.

As Sidorak flew from the Land of the Rising Sun back to the United States in 2015, he reflected on the pilgrimage.

"What was done cannot be undone. However, the first victims of the atomic age on August 6 and August 9 seventy years ago, the people of Hiroshima and Nagasaki, will not have died in vain, if nuclear weapons are never used again."