Donald Trump had to pause a phone call with Russian President Vladimir Putin to ask an aide what the nuclear treaty between the countries was.

The leaders were discussing the 2010 treaty New START, which caps the amount of deployed strategic nuclear warheads for both countries, when Trump had to stop the 28 January phone call to ask what the treaty was.

After he was offered an explanation, Trump then resumed the conversation to denounce the treaty, claiming Russia had benefitted from it more than the US.

The US president has discussed the treaty previously, incorrectly referring to it as START Up during his election campaign and claiming Russia had "outsmarted" the US during negotiation of the deal, The Independent reported.

The Republican's reported claims that the deal was a bad negotiation by former president Barack Obama has prompted concerns that a new nuclear arms race could be on the cards.

New START requires both the US and Russia to cut their deployed strategic nuclear warheads to 1,550 by February 2018, with the deal due to be renewed in 2021.

A failure to renew the deal or to agree new targets for reduction will allow both countries to develop nuclear arms as they see fit.

The discussion on New START does not appear in the White House's log and it declined to comment on the issue, Reuters reported.

The conversation also raised concerns over how prepared Trump is to speak with foreign leaders – with presidents usually briefed on the issues to be discussed ahead of phone calls.

It is not clear whether Trump was ineffectively briefed to speak with Putin or whether he needed additional information on the subject of New START.