A group of former officials from the U.S., Russia, Germany and the United Kingdom are calling on President Trump and Russian President Vladimir Putin to collaborate on issues involving nuclear deterrence.

The group, which includes former Sen. Sam Nunn, D-Georgia, is urging Mr. Trump and Putin to take advantage of their first opportunity to meet in Hamburg, Germany next month to coordinate on a series of steps that reduce nuclear and other military risks and to prevent terrorist attacks.

"The starting point could be a new Presidential Joint Declaration by the United States and the Russian Federation declaring that a nuclear war cannot be won and must never be fought," the group wrote in the letter.

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Other steps both leaders could take, the group argued, involve increasing communication between their militaries, preventing terrorist groups like the Islamic State in Iraq and Syria (ISIS) from acquiring radiological and nuclear material and reaching informal understandings about cyber dangers.

The other officials who signed the letter are former British Defense Secretary Des Browne, former German Ambassador to the U.S. Wolfgang Ischinger and former Russian Foreign Minister Igor Ivanov.

Mr. Trump will head to Hamburg for the G-20 summit late next week. Putin is expected to attend as well.